drm/msm/dpu: unwind async commit handling

It attempted to avoid fps drops in the presence of cursor updates.  But
it is racing, and can result in hw updates after flush before vblank,
which leads to underruns.

Signed-off-by: Rob Clark <robdclark@chromium.org>
Reviewed-by: Sean Paul <sean@poorly.run>
Signed-off-by: Rob Clark <robdclark@chromium.org>