msm: camera: Extend scope of mutex for flush request

There may be chance that flush and release can happen at the same
time. In this situation, flush request can get the control although
release is unlinking it. This change increase scope of mutex for
flush operation to make sure operation happens correctly.

Change-Id: I08520e1133f6fc903eb94463ee384e5ee8d848e2
Signed-off-by: Jigarkumar Zala <jzala@codeaurora.org>