drm/msm/sde: check in video encoder irq if ctl was flushed

Add logic in the SDE video encoder vblank irq handler to verify
that the hardware actually flushed before decrementing the
pending flush counter. This prevents incorrectly signaling that
the buffer was flipped.

Change-Id: Idd723f80a8a7f5f596654a784c3a76abe75c4f19
Signed-off-by: Lloyd Atkinson <latkinso@codeaurora.org>