drm: Kerneldoc for drm_mode_config_funcs

The meat here is definitely the detailed specs for what atomic_check
and atomic_commit are supposed to do.

And another candidate for a core vfunc that should be in a helper really
(output_poll_changed this time around).

v2: Feedback from Eric on irc:
- spelling fixes.
- spec what async should do
- copy the event related paragraphs from page_flip and adjust
- make it clear that a successful async commit is not allowed to leave
  the pipe dead or disabled.

v3: Use FIXME comments to annotate functions that we should move to
some helpers.

v4: Suggestions from Thierry.

Cc: Eric Anholt <eric@anholt.net>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1449218769-16577-22-git-send-email-daniel.vetter@ffwll.chReviewed-by: Thierry Reding <treding@nvidia.com>