msm: mdss: handle race condition between ov_off and ov_commit

Handle race condition between ov_off and ov_commit by
acquiring the ov_lock in off API. This gives the exclusive
behavior between ov_off and ov_commit call.

Change-Id: Iaa13f9e5991a53feff12b8efaec497cae49d28d1
Signed-off-by: Dhaval Patel <pdhaval@codeaurora.org>