msm: camera: isp: Use a lock when getting a payload for bus IRQs

The payloads for VFE bus IRQs are saved on a list when not in use.
Protect this list with a lock since the top- and bottom-halves can
access this list at the same time.

Change-Id: I86fb4186684de221b2bc4d070505c07988cc1b31
Signed-off-by: Vishalsingh Hajeri <vhajeri@codeaurora.org>