drm/fb-helper: Prepare to move out commit code

This makes the necessary changes so the commit code can be moved out to
drm_client as-is in the next patch. It's split up to ease review.

Signed-off-by: Noralf Trønnes <noralf@tronnes.org>
Reviewed-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20190531140117.37751-4-noralf@tronnes.org