drm/nouveau: Skip vga_fini on non-PCI device

As with vga_init, this function doesn't make sense on non-PCI devices,
and the Thunderbolt check in it dereferences a NULL pointer in that
case. Add some code to skip this function when the device is not a PCI
device.

Signed-off-by: Mikko Perttunen <mperttunen@nvidia.com>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>