backend/drm: skip reset after VT switch if possible
If all connectors and planes already have the right CRTC set, or are disabled, we can skip the device-wide reset after a VT switch.
I've contemplated using a more fine-grained logic to only reset the connectors, CRTCs and planes that need to be migrated. However, writing a correct algorithm for this would be quite involved, and it doesn't seem worth the trouble anyways.