mdss: rotator: remove ov_lock for rotator sessions

The ov_lock is meant to protect overlay/display pipeline.
Rotator has a different lock which synchronizes its resources.
Remove the ov_lock for rotator ioctl calls.

Change-Id: Ie38bb796c3430951636f8c6888bca4cfc8194aaf
Signed-off-by: Adrian Salido-Moreno <adrianm@codeaurora.org>