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

Signed-off-by: Ben Skeggs <bskeggs@redhat.com>

drivers/gpu/drm/nouveau/nouveau_object.c

@@ -870,16 +870,22 @@ void
 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);