drm/msm: subclass work object for vblank events

msm maintains a separate structure to define vblank
work definitions and a list to track events submitted
to the workqueue. We can avoid this redundant list
and its protection mechanism, if we subclass the
work object to encapsulate vblank event parameters.

changes in v2:
	- subclass optimization on system wq (Sean Paul)
changes in v3:
	- none
changes in v4:
	- move flush_workqueue before irq uninstall
changes in v5:
	- none

Signed-off-by: Jeykumar Sankaran <jsanka@codeaurora.org>
Signed-off-by: Sean Paul <seanpaul@chromium.org>