msm_fb: display: add mutex to video_off

It is possible  that pipe_commit still be issued by
frame work after suspend had been started. Therefore
mutex lock within video_off is necessary to prevent
race condition from happening between these two threads.

Change-Id: I57f51744c1654d6def8664c492035631a5b7081e
Signed-off-by: Kuogee Hsieh <khsieh@codeaurora.org>