disp: msm: sde: avoid wb commit before cwb disable

Add support to detect if a writeback commit is received
while cwb commit is going on. Allow writeback commit
only once the cwb commit is completed and cwb is
disabled.

Change-Id: I020659ed63f9ff660a905938a92a6ed6d07a4917
Signed-off-by: Krishna Manikandan <mkrishn@codeaurora.org>