drm/nvc0: reject the notifier_alloc ioctl

nouveau_ioctl_notifier_alloc(struct drm_device *dev, void *data,

			     struct drm_file *file_priv)

{

	struct drm_nouveau_private *dev_priv = dev->dev_private;

	struct drm_nouveau_notifierobj_alloc *na = data;

	struct nouveau_channel *chan;

	int ret;

	/* completely unnecessary for these chipsets... */

	if (unlikely(dev_priv->card_type >= NV_C0))

		return -EINVAL;

	chan = nouveau_channel_get(dev, file_priv, na->channel);

	if (IS_ERR(chan))

		return PTR_ERR(chan);