drm: Document drm_plane_helper_funcs

Plus related hooks used to do atomic plane updates since they only
really make sense as a package.

v2: Suggestions from Thierry.
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1449218769-16577-24-git-send-email-daniel.vetter@ffwll.chReviewed-by: Thierry Reding <treding@nvidia.com>