drm/atomic-helper: Annotate a bunch more RETURNS: sections

kernel-doc wants a : at the end.

Acked-by:  <jani.nikula@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1465388359-8070-27-git-send-email-daniel.vetter@ffwll.ch