So this is finally the integration of the crtc and plane helper
interfaces into the atomic helper functions.

In the check function we now have a few steps:

- First we update the output routing and figure out which crtcs need a
  full mode set. Suitable encoders are selected using ->best_encoder,
  with the same semantics as the crtc helpers of implicitly disabling
  all connectors currently using the encoder.

- Then we pull all other connectors into the state update which feed
  from a crtc which changes. This must be done do catch mode changes
  and similar updates - atomic updates are differences on top of the
  current state.

- Then we call all the various ->mode_fixup to compute the adjusted
  mode. Note that here we have a slight semantic difference compared
  to the crtc helpers: We have not yet updated the encoder->crtc link
  when calling the encoder's ->mode_fixup function. But that's a
  requirement when converting to atomic since we want to prepare the
  entire state completely contained with the over drm_atomic_state
  structure. So this must be carefully checked when converting drivers
  over to atomic helpers.

- Finally we do call the atomic_check functions on planes and crtcs.

The commit function is also quite a beast:

- The only step that can fail is done first, namely pinning the
  framebuffers. After that we cross the point of no return, an async
  commit would push all that into the worker thread.

- The disabling of encoders and connectors is a bit tricky, since
  depending upon the final state we need to select different crtc
  helper functions.

- Software tracking is a bit clarified compared to the crtc helpers:
  We commit the software state before starting to touch the hardware,
  like crtc helpers. But since we just swap them we still have the old
  state (i.e. the current hw state) around, which is really handy to
  write simple disable functions. So no more
  drm_crtc_helper_disable_all_unused_functions kind of fun because
  we're leaving unused crtcs/encoders behind. Everything gets shut
  down in-order now, which is one of the key differences of the i915
  helpers compared to crtc helpers and a really nice additional
  guarantee.

- Like with the plane helpers the atomic commit function waits for one
  vblank to pass before calling the framebuffer cleanup function.

Compared to Rob's helper approach there's a bunch of upsides:

- All the interfaces which can fail are called in the ->check hook
  (i.e. ->best_match and the various ->mode_fixup hooks). This means
  that drivers can just reuse those functions and don't need to move
  everything into ->atomic_check callbacks. If drivers have no need
  for additional constraint checking beyong their existing crtc
  helper callbacks they don't need to do anything.

- The actual commit operation is properly stage: First we prepare
  framebuffers, which can potentially still fail (due to memory
  exhausting). This is important for the async case, where this must
  be done synchronously to correctly return errors.

- The output configuration changes (done with crtc helper functions)
  and the plane update (using atomic plane helpers) are correctly
  interleaved: First we shut down any crtcs that need changing, then
  we update planes and finally we enable everything again. Hardware
  without GO bits must be more careful with ordering, which this
  sequence enables.

- Also for hardware with shared output resources (like display PLLs)
  we first must shut down the old configuration before we can enable
  the new one. Otherwise we can hit an impossible intermediate state
  where there's not enough PLLs (which is the point behind atomic
  updates).

v2:
- Ensure that users of ->check update crtc_state->enable correctly.
- Update the legacy state in crtc/plane structures. Eventually we want
  to remove that, but for now the drm core still expects this (especially
  the plane->fb pointer).

v3: A few changes for better async handling:

- Reorder the software side state commit so that it happens all before
  we touch the hardware. This way async support becomes very easy
  since we can punt all the actual hw touching to a worker thread. And
  as long as we synchronize with that thread (flushing or cancelling,
  depending upon what the driver can handle) before we commit the next
  software state there's no need for any locking in the worker thread
  at all. Which greatly simplifies things.

  And as long as we synchronize with all relevant threads we can have
  a lot of them (e.g. per-crtc for per-crtc updates) running in
  parallel.

- Expose pre/post plane commit steps separately. We need to expose the
  actual hw commit step anyway for drivers to be able to implement
  asynchronous commit workers. But if we expose pre/post and plane
  commit steps individually we allow drivers to selectively use atomic
  helpers.

- I've forgotten to call encoder/bridge ->mode_set functions, fix
  this.

v4: Add debug output and fix a mixup between current and new state
that resulted in crtcs not getting updated correctly. And in an
Oops ...

v5:
- Be kind to driver writers in the vblank wait functions.. if thing
  aren't working yet, and vblank irq will never come, then let's not
  block forever.. especially under console-lock.
- Correctly clear connector_state->best_encoder when disabling.
  Spotted while trying to understand a report from Rob Clark.
- Only steal encoder if it actually changed, otherwise hilarity ensues
  if we steal from the current connector and so set the ->crtc pointer
  unexpectedly to NULL. Reported by Rob Clark.
- Bail out in disable_outputs if an output currently doesn't have a
  best_encoder - this means it's already disabled.

v6: Fixupe kerneldoc as reported by Paulo. And also fix up kerneldoc
in drm_crtc.h.

v7: Take ownership of the atomic state and clean it up with
drm_atomic_state_free().

v8 Various improvements all over:
- Polish code comments and kerneldoc.
- Improve debug output to make sure all failure cases are logged.
- Treat enabled crtc with no connectors as invalid input from userspace.
- Don't ignore the return value from mode_fixup().

v9:
- Improve debug output for crtc_state->mode_changed.

v10:
- Fixup the vblank waiting code to properly balance the vblank_get/put
  calls.
- Better comments when checking/computing crtc->mode_changed

v11: Fixup the encoder stealing logic: We can't look at encoder->crtc
since that's not in the atomic state structures and might be updated
asynchronously in and async commit. Instead we need to inspect all the
connector states and check whether the encoder is currently in used
and if so, on which crtc.

v12: Review from Sean:
- A few spelling fixes.
- Flatten control flow indent by converting if blocks to early
  continue/return in 2 places.
- Capture connectors_for_crtc return value in int num_connectors
  instead of bool has_connectors and do an explicit int->bool
  conversion with !!. I think the helper is more useful for drivers if
  it returns the number of connectors (e.g. to detect cloning
  configurations), so decided to keep that return value.

Cc: Sean Paul <seanpaul@chromium.org>
Cc: Paulo Zanoni <przanoni@gmail.com>
Cc: Rob Clark <robdclark@gmail.com>
Reviewed-by: Sean Paul <seanpaul@chromium.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>