backends/drm: turn displays off before suspend
This is done for three reasons:
- it allows to handle spurious wakeups of devices better in the future, by keeping the screen off when it wakes, and just having powerdevil suspend the device again if it's still idle. This MR still enables them again in KWin, but that can easily be changed in a follow up.
- it makes the suspend process look faster to the user
- we can show a nice animation (currently just fade) when suspending
There's a caveat to the animation... we immediately blank the screen for the lockscreen, so you can't actually see any animations if "lock after waking from sleep" is enabled. Idk what to do about that one exactly, but it doesn't have to be taken care of here.