drm/atomic: Return commit in drm_crtc_commit_get for better annotation

This will allow code to do x->commit = drm_crtc_commit_get(commit),
making it clearer where references are used.

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20170904104838.23822-5-maarten.lankhorst@linux.intel.com
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>