drm/msm/dpu: add real wait_for_commit_done()

Just waiting for next vblank isn't ideal.. we should really be looking
at the hw FLUSH register value to know if there is still an in-progress
flush without stalling unnecessarily when there is no pending flush.

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