drm: Add kerneldoc for drm_framebuffer_funcs

While typing these I noticed that ->dirty is a bit a can of worms
and even supports blt/fill semantics ... shocked me a bit.

Oh well it's defined in a way that nothing bad (just a bit of inefficiency)
will happen for drivers which supports this. So I didn't bother copying
the detailed spec into the new kerneldoc.

v2: Suggestions from Thierry.

Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1449218769-16577-21-git-send-email-daniel.vetter@ffwll.ch


Reviewed-by: Thierry Reding <treding@nvidia.com>