drm/msm: add subdriver callback for single kickoff

Add a callback that will give subdrivers a single point where
flush to hardware trigger is appropriate in all commit
callflows; after bridge pre-enable and encoder enable, but
before bridge enable. Current flush to hardware trigger is
spread across different code paths depending on whether the
commit is doing an enable or just a page flip.

Change-Id: If3c9383f176af90b154dd02b338017c48e2fbc70
Signed-off-by: Lloyd Atkinson <latkinso@codeaurora.org>