msm: camera: isp: Protect the evt_handler with lock

Extend the rw_lock to include th_procesing(), else unsunscribe_irq()
can be called and evt_handler will be freed, causing use after free
in th_processing on evt_handler.
Also, remove asymmetric call to unsubscribe in cam_vfe_stop(). For
VFE Out resources, we do not call subscribe in cam_vfe_start(), so no
need to call unsubscribe.

Change-Id: Ie45a7244645d364b727948fd30f830c6d958ff8a
Signed-off-by: Harsh Shah <harshs@codeaurora.org>