udev: Try to detect virtual graphics and EFI better

With EFI and simpledrm we may end up booting without a card0 but
with a card1 drm device. This defeats GDM's simplistic hybrid
graphics check.

Also, our detection for virtio graphics wasn't working so well, because
it wrote the results into an environment variable of a non-parent
device.

This commit gets the detection logic working better for virt/efi setups.

Closes: https://gitlab.gnome.org/GNOME/gdm/-/issues/832