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: I8305be685418938568f7920240dba0369f895715
Signed-off-by: Lynus Vaz <lvaz@codeaurora.org>