drm/nouveau/hwmon: return EINVAL if the GPU is powered down for sensors reads

fixes bogus values userspace gets from hwmon while the GPU is powered down

Signed-off-by: Karol Herbst <kherbst@redhat.com>
Reviewed-by: Rhys Kidd <rhyskidd@gmail.com>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>