drm: msm: sde: kernel needs to monitor bootloader's flush done

Once kernel sends splash stop signal to bootloader, it
needs to listen to bootloader's cocurrent flush to
hardware is done, otherwise, one race condition issue
for mixer clear and setup messed may happen.

Change-Id: I7b603b9acc0dd0af07da3c5afee052007432b7fb
Signed-off-by: Guchun Chen <guchunc@codeaurora.org>
Signed-off-by: Rahul Sharma <rahsha@codeaurora.org>