fbdev: msm: Increment commit count during null commit

Calling overlay_kickoff during overlay_off path without
incrementing commit_cnt results in release fence timeline
getting incremented and causes early fence signalling in
subsequent commits leading to flicker. To avoid this,
increment commit_cnt before null commit is initiated.

Change-Id: I7ec482bfc192a58fa8819633a7830f440b3bd91b
Signed-off-by: Nirmal Abraham <nabrah@codeaurora.org>