workspace: fix the dpms input event filter sometimes being wrongly deleted
Before, it could break in the following scenario:
- drm backend emits aboutToTurnOff
- workspace creates the dpms filter
- workspace gets some event that triggers it to update outputs
- maybeDestroyDpmsFilter sees that the output is on and deletes the dpms filter
- the drm backend actually turns the screen off
In that case, the dpms input event filter is gone and the output off. Unless the dpms mode gets changed again by some other process, the output would now stay off forever.