msm: mdss: fix race condition of DSI off and status check.

fb_event_callback cancels the work queue when it receives
the power down event. However, this event occurs after the
panel & DSI have been turned off. So if the check_status
function was being called right when the panel is being
turned off, we have a race conditon. Hence add offlock
mutex to ensure that bta status check is done when dsi
is being turned off.

Change-Id: Ie341c002c4544376bda2940fa65c6eb13997ae17
Signed-off-by: Vineet Bajaj <vbajaj@codeaurora.org>