backend/drm: refuse to switch CRTC for enabled connector
match_obj() might return a configuration where the CRTC for an enabled connector is switched to another one.
We don't support this correctly: the wlr_output common code would need to query again the supported formats, re-allocate the swapchain, etc.
What's more, the kernel doesn't even support this 1: it requires planes to be disabled to change their CRTC, it rejects commits directly switching the CRTC used by a plane.