msm: mdss: Prevent race condition between dsi off and bta status check

fb_event_callback() is what is responsible for cancelling the
delayed_work when it gets the FB_BLANK_POWERDOWN 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 check
shutdown flag to avoid bta status check when dsi is being turned off.

Change-Id: I103bafb4ab63be83c47dfd6b3bad06d36704ff68
Signed-off-by: Jayant Shekhar <jshekhar@codeaurora.org>