drm/nouveau: Fix backlight control on PPC machines with an internal TMDS panel.

This commit fixes fdo bug 29685.

Reported-by: Vlado Plaga <rechner@vlado-do.de>
Signed-off-by: Francisco Jerez <currojerez@riseup.net>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>