disp: msm: sde: add mutex lock to handle ppdone timeout usecases

The irq register and unregister calls in ppdone timeout
path can result in race condition which can lead to
scenarios where irqs will be enabled with no
callback registered. Add vblank mutex lock to avoid
such scenarios.

Change-Id: I02b07b773d6b9a28c3af431eb3d8314292769f12
Signed-off-by: Krishna Manikandan <mkrishn@codeaurora.org>