drm/msm/sde: Add property for AD vsync count

On command mode panel, vsync notification will triggered for each
TE, not actual vsync commit. This change adds an AD property to
track vsync event count for AD, so that AD feature can converge
with the correct display commit count.

Change-Id: I6c87ce989203f8bb121723465c661e2b2f27c179
Signed-off-by: Ping Li <pingli@codeaurora.org>