msm: mdss: fix race condition between commit thread and power off

There is a deadlock in use cases where the power off
is called concurrently with the overlay kickoff from
different threads. To avoid this condition, during
power off sequence we first make sure to stop the
fb display thread before the power off sequence is
handled.

Change-Id: I708143206b914f6f72c440c4dc3a1c73fceea6b2
Signed-off-by: Ingrid Gallardo <ingridg@codeaurora.org>