drm/nv50: wait for pgraph to idle before unloading the context

This should fix the problem with gpu hangs people have had when closing
channels.

Signed-off-by: Maarten Maathuis <madman2003@gmail.com>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>

drivers/gpu/drm/nouveau/nv50_graph.c

@@ -282,6 +282,7 @@ nv50_graph_unload_context(struct drm_device *dev)
 		return 0;
 	inst &= NV50_PGRAPH_CTXCTL_CUR_INSTANCE;
 
+	nouveau_wait_for_idle(dev);
 	nv_wr32(dev, 0x400500, fifo & ~1);
 	nv_wr32(dev, 0x400784, inst);
 	nv_wr32(dev, 0x400824, nv_rd32(dev, 0x400824) | 0x20);