drm: document drm_crtc_funcs

drm: document drm_crtc_funcs

And merge any docbook we have into the kerneldoc comments.

Since it's a legacy entry point with only two implementation (one each
in atomic and legacy crtc helpers) I've made the documentation for
set_config fairly sparse - no one should ever need to look at this
again, all the ABI we have is baked into code.

For ->page_flip otoh I kept all the extensive docs from the docbook
and even extended it where it was lacking: Currently we have a pile of
legacy page_flip implemantations, and even for atomic drivers there's
not yet a standard implementation in the helpers. Which means every
driver needs to implement this itself, and precise specs are really
valuable.

Otherwise there's just cursor, which really just boils down to "use at
least universal planes". And gamma tables (where we have a bit a mess
with the fbdev helper gamma hooks).

v2: Spelling fixes (Eric).

v3: Suggestions from Thierry.

Cc: Eric Anholt <eric@anholt.net>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1449218769-16577-20-git-send-email-daniel.vetter@ffwll.ch