video: msm: Unlock mutex before update notify

When update notify functions holds the entry_mutex, it blocks other MDP
ioctls from progressing causing a system freeze. Unlock the lock before
calling to prevent this.

Change-Id: I146b0a92ec844d90f4a8688bd77a38e298c2b9c0
Signed-off-by: Carl Vanderlip <carlv@codeaurora.org>