drm/nouveau: fail runtime pm properly.

If we were on a non-optimus device, we'd return -EINVAL, this would
lead to the over engineered runtime pm system to go into an error
state, subsequent get_sync's would fail, so we'd never be able
to open the device again.

(like really get_sync shouldn't fail if the device isn't powered
down).

Signed-off-by: Dave Airlie <airlied@redhat.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>