drm/nouveau/bl: fix backlight regression

[ Upstream commit 9e75dc61eaa9acd1bff83c3b814ac2af6dc1f64c ]

Fixes: 3c66c87d ("drm/nouveau/disp: remove hw-specific customisation
of output paths")
Suggested-by: Ben Skeggs <skeggsb@redhat.com>
Signed-off-by: Karol Herbst <kherbst@redhat.com>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>
Signed-off-by: Sasha Levin <alexander.levin@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>