drm: Include drm_crtc_helper.h in DocBook

There is already a section that describes the helpers implemented by
this module. Add the kerneldoc-generated structure descriptions to this
section.

While at it, add missing kerneldoc for the structures to avoid warnings
when generating the documentation.

Signed-off-by: Thierry Reding <treding@nvidia.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>