msm: camera: isp: Move the lock to executing function

Move the lock responsibility from calling method to the place
where the mutually exclusive code executes. This will prevent
potential errors of unprotected calls from calling places.

Change-Id: Ibdd7080774ff686b4fc96d1d2d020564b367b79a
Signed-off-by: Harsh Shah <harshs@codeaurora.org>