[display] Start ui before ramping up display backlight. JB#33873
[display] Start ui before ramping up display backlight. JB#33873

The display backlight ramp up is started after mce believes the frame
buffer device has been powered up so that ithappens in parallel with the
ipc that allows compositor to draw on screen again. Now, if the compositor
startup is slow enough and/or mce is wrong about display hw power state,
this can lead to effective backlight level staying at zero and display
remaining blanked even though there is active ui on it.

Wait for compositor to acknowledge updates enabled state change before
ramping up the backlight level.

mlehtima/mce,mlehtima/mce,spiiroin/mce,spiiroin/mce,maikelwever/mce,mlehtima/mce,mlehtima/mce,maikelwever/mce,maikelwever/mce,maikelwever/mce,spiiroin/mce,spiiroin/mce