Xorg binds RandR providers to screen 0 implicitly
This is a problem when there's a UDL device on its own screen with explicit xorg.conf.d configuration.
A CRTC-less device output from the primary GPU can be assigned along with the UDL device to the same screen to set up Prime or reverse Prime with the proper Device and GPUDevice settings.
But since everything is bound to screen 0, DISPLAY=:0.n xrandr --listproviders (n >= 1) doesn't show any providers and "xrandr --setprovider ..." on the same screen refuses to work because there are no providers.
!337 (merged) is an attempt to fix this use case and use the configured screen number instead of 0.