drm: Document drm_encoder/crtc_helper_funcs

Mostly this is about all the callbacks used for modesets by both legacy
CRTC helpers and atomic helpers and I figured it doesn't make all that
much sense to split this up.

v2: Suggestions from Thierry.
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1449218769-16577-28-git-send-email-daniel.vetter@ffwll.chReviewed-by: Thierry Reding <treding@nvidia.com>