msm: camera: actuator: Add mutex lock during apply and flush request

apply_request and flush_request needs to be protected by mutex in
order to make sure that same i2c index is not being use at the same
time. This change adds mutex lock for that critical section.

Change-Id: I5c3f10f902b3efe1c96cc88ccafb05f2782ad55f
Signed-off-by: Jigarkumar Zala <jzala@codeaurora.org>