backends/drm: ensure modeset properties are reset properly

Without this, all modeset commits fail on setups with a VR headset because some properties aren't reset after enabling it and disabling it again. This is effectively the same problem as !2564 (merged) but with the states reversed.

I plan to do a general rework of how the modeset stuff works that should solve this better, but for now (and for backporting) this should fix it.