drm/doc: Add GEM/CMA helpers to kerneldoc

Most of the functions already have the beginnings of kerneldoc comments
but are using the wrong opening marker. Use the correct opening marker
and flesh out the comments so that they can be integrated with the DRM
DocBook document.

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