drm/msm/dpu: move vblank events to complete_commit()

We could get a vblank event racing with the current atomic commit,
resulting in sending the pageflip event to userspace early, causing
tearing.  On the other hand, complete_commit() ensures that the
pending flush is complete.

Signed-off-by: Rob Clark <robdclark@chromium.org>
Reviewed-by: Abhinav Kumar <abhinavk@codeaurora.org>