Draft: backend/drm: use a single commit to restore state after VT switch
Let's keep this bit in a separate MR from !4548 (merged) because I'm not yet sure the logic here is quite correct...
Unfortunately things are more complicated than they need to be because the kernel will reject commits with DRM_MODE_PAGE_FLIP_EVENT
including CRTCs that are currently off and are still off in the new state...
Alternatively we could drop the DRM_MODE_PAGE_FLIP_EVENT
flag and manually restart the compositor's render loop on outputs which are enabled.