After all relevant pipes are disabled and after we've updated all the
state with the staged state, but before we call the per-crtc
->mode_set functions there's a very natural point to set up any
shared/global resources like
- shared plls (obviously only the setup, the enabling needs to be
  separately handling with a separate refcount)
- global watermark state like the DSPARB on gmch platforms
- workaround bits that depend upon the exact global output
  configuration
- enabling the right set of refclocks
- enabling/disabling manual power wells.

Now for a lot of these things we can't move them into this function
yet, most often because we only compute the required information in
the per-crtc ->mode_set callback. Which is too late. But due to a
bunch of reasons (check-only atomic modeset, fastboot&hw state checks,
...) we need to separate the computation of that state from the actual
hw frobbery anyway. So we can move things into this new callback step-
by-step.

Others can't be moved here (or implemented at all) because our code
lacks the smarts to properly update them. E.g. the DSPARB can only be
updated when all pipes are disabled, so if we decide to change it's
value, we need to disable _all_ pipes. The infrastructure for that is
already in place (with the various pipe masks that driver the modeset
logic). But again we need to move a few things out of ->mode_set
first before we can even implement the correct decision making.

In any case, we need to start somewhere, so let's start with the
callback: Some small follow-up patches will make immediate good use of
it.
Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>