drm: Pull drm_crtc.h into the kerneldoc template

While writing atomic docs I've noticed that I don't get any errors
for my screw-ups in drm_crtc.h. Fix this immediately.

This just does the bare minimum to get starts, lots of stuff isn't
properly documented yet unfortunately.

v2: Fix adjacent spelling error Sean noticed.

Reviewed-by: Sean Paul <seanpaul@chromium.org>
Cc: Sean Paul <seanpaul@chromium.org>
Reviewed-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>