drm/nv50-nvc0/vm: support unsnooped system memory

v2 (Emil Velikov <emil.l.velikov@gmail.com>):
- Fixed a regression on certain nv50 IGP due to not passing the correct
  target type to nv50_vm_addr()

Signed-off-by: Ben Skeggs <bskeggs@redhat.com>
Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>
Tested-by: Johannes Obermayr <johannesobermayr@gmx.de>