drm/nouveau: fix backlight mask on ppc powerbook

This code was originally moved to using nv_mask by d31e078d. This
should not have any actual effect since the mask isn't applied to the
value.

Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>