Revert "drm/nouveau: inform userspace of new kernel subchannel requirements"

This reverts commit a81f15499887d3f9f24ec70bb9b7e778942a6b7b.

Gah, we have a released userspace component using fixed subc assignment
that conflicts with this.  To avoid breaking ABI this needs to be
reverted.

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