msm: mdss: disable dsi status check when display is off

There was some racing condition where the dsi status check
thread is called when the display is off.   This makes sure
the dsi status check is not invoked when display is off.

Change-Id: I6865abe23ca7adf918a19499d3839d6664fc29fc
Signed-off-by: Xiaoming Zhou <zhoux@codeaurora.org>