drm/msm/sde: correct to use sync vblank request for commit

Current sde_wait_for_commit_done is calling asynchronous
drm_crtc_vblank_get/put to enable/disable vblank interrupt.
However, drm_crtc_vblank_get/put submit the vblank request
to a workqueue for later execution. As a result, vblank
irq is not get/put properly prior/after the commit operation
in sde_wait_for_commit_done.

Replace async drm_crtc_vblank_get/put with sync sde_crtc_vblank,
and use reference count in encoder to enable/disable interrupts.

Change-Id: Iccc83983d56b75717eda9c401a54dfe77e945fed
Signed-off-by: Alan Kwong <akwong@codeaurora.org>