disp: msm: queue vblank work on event thread

This change queues vblank work to event thread rather than
the display thread. This allows vblank events to be processed
without potentially being delayed by crtc commit being processed.

Change-Id: I9cbf9b3de646716e4698af9f338a7140346b65fd
Signed-off-by: Samantha Tran <samtran@codeaurora.org>