drm/nouveau: Check backlight IDs are >= 0, not > 0

commit dc854914999d5d52ac1b31740cb0ea8d89d0372e upstream.

Remember, ida IDs start at 0, not 1!

Signed-off-by: Lyude Paul <lyude@redhat.com>
Reviewed-by: Karol Herbst <kherbst@redhat.com>
Cc: stable@vger.kernel.org
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>