And also put the overview section into the KMS Properties part of the
docs, instead of randomly-placed within the helpers - this is part of
the uabi.

With this patch I think drm_crtc.[hc] is cleaned up and entirely
documented.
Reviewed-by: NChris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: NDaniel Vetter <daniel.vetter@intel.com>