msm: camera: isp: Remove recurrent mutex lock call

There is a deadlock resulting from trying to acquire the same
mutex lock in shutdown call. Remove one of the lock acquires
to resolve this.

Change-Id: Id724d5c031b5d56e76a0e3efc50282b4a74b39a5
Signed-off-by: Harsh Shah <harshs@codeaurora.org>