drm/nv50-/kms: allow disabling of gpu scaling on fixed panels

The hilarious part is that, under X, this won't work anyway because the
server decides to construct its own modes for some reason.

Tested with modetest, which isn't quite as insane.  I'd hope that
wayland is more sensible.

Signed-off-by: Ben Skeggs <bskeggs@redhat.com>