drm/msm/sde: remove redundant CRTC event caching

Currently both sde_crtc_atomic_begin() and
sde_crtc_atomic_flush() add the CRTC state event
to the cached sde_crtc->event.

This has a potential NULL ptr issue in the
case of vblank event firing in between sde_crtc_atomic_begin() and
sde_crtc_atomic_flush() because the upstream DRM vblank API
send_vblank_event() doesn't consider the case when the VBLANK
interrupt could have already freed any pending vblank events.

Remove the caching from sde_crtc_atomic_begin() to avoid this
condition.

Also make sure that a page_flip event was indeed submitted before
signaling the complete_flip() by setting a PENDING_FLIP flag right
after HW flush.

Change-Id: Ib201d2851e57bf22ec1f00814fc2e4dd2f35bfa1
Signed-off-by: Abhinav Kumar <abhinavk@codeaurora.org>
Signed-off-by: Yunyun Cao <yunyunc@codeaurora.org>