msm: mdss: fix a dead-lock issue on 8x10

The shutdown function involves two seperate steps: shut down
the dma and unset the overlay.   Due to the use of the mutex,
a UI update thread can comes in between the two steps.   Futher,
the UI update thread does not hold the mutex while trying to
check the display status.   This results a dead-lock: UI update
thread tries to wait for vsync event, while the display has already
been shut-down.

Change-Id: Ib1242c60605a85441af78dcdc75a69a209dec7d1
Signed-off-by: Xiaoming Zhou <zhoux@codeaurora.org>