drm/nouveau: fix null pointer deref on pre-nv50 chipsets

nouveau_gpuobj_channel_takedown(struct nouveau_channel *chan)

{

	struct drm_device *dev = chan->dev;

	struct drm_nouveau_private *dev_priv = dev->dev_private;

	int i;

	NV_DEBUG(dev, "ch%d\n", chan->id);

	if (dev_priv->card_type >= NV_50) {

		struct nv50_display *disp = nv50_display(dev);

		for (i = 0; i < 2; i++) {

		struct nv50_display_crtc *dispc = &nv50_display(dev)->crtc[i];

			struct nv50_display_crtc *dispc = &disp->crtc[i];

			nouveau_bo_vma_del(dispc->sem.bo, &chan->dispc_vma[i]);

		}

		nouveau_vm_ref(NULL, &chan->vm, chan->vm_pd);

		nouveau_gpuobj_ref(NULL, &chan->vm_pd);

	}

	if (drm_mm_initialized(&chan->ramin_heap))

		drm_mm_takedown(&chan->ramin_heap);