drm: Update drm_plane_funcs kerneldoc

drm: Update drm_plane_funcs kerneldoc

- Merge the docbook into the kerneldoc comments.

- Spec in detail the precise semantics of the callbacks.

- For consistency in wording and easier review roll out kerneldoc also
  for crtc, encoder and connector for the standard hooks they share
  with planes.

v2: Suggestions from Thierry.

Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1449218769-16577-8-git-send-email-daniel.vetter@ffwll.ch
Reviewed-by: Thierry Reding <treding@nvidia.com>