Since we're about to use this in nouveau_backlight.c. Same thing as
DRM_WARN_ONCE, DRM_INFO_ONCE, etc...
Signed-off-by: Lyude Paul <lyude@redhat.com>
Cc: Karol Herbst <kherbst@redhat.com>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>