msm: camera: Synchronize v4l2 subscribe and unsubscribe event in camera.c

If same event is unsubscribed before v4l2_event_subscribe returned,
Then function v4l2_event_subscribe have possibility use-after-free.

Serialize msm_subscribe_event and msm_unsubscribe_event to
prevent parallel invocation of v4l2_event_subscribe and
v4l2_event_unsubscribe.

Change-Id: Ia28cfb9d46550d58221f157337a1468f524753e3
Signed-off-by: Trishansh Bhardwaj <tbhardwa@codeaurora.org>