drm/nouveau/kms/nv50-: Explicitly check DPCD backlights for aux enable/brightness

Since we don't support hybrid AUX/PWM backlights in nouveau right now,
let's add some explicit checks so that we don't break nouveau once we
enable support for these backlights in other drivers.

Reviewed-by: Karol Herbst <kherbst@redhat.com>
Signed-off-by: Lyude Paul <lyude@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211105183342.130810-3-lyude@redhat.com