msm: camera: Prevent use-after-free in v4l2_event_subscribe.

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: I321fc66a9acbfb6e511b553be4cedffd26a558db
Signed-off-by: Trishansh Bhardwaj <tbhardwa@codeaurora.org>