drm/nv40/mpeg: write magic value to channel object to make it work

Looks like the rewrite in commit ebb945a9 ("drm/nouveau: port all
engines to new engine module format") missed that one little detail.
Signed-off-by: NIlia Mirkin <imirkin@alum.mit.edu>
Signed-off-by: NBen Skeggs <bskeggs@redhat.com>

drivers/gpu/drm/nouveau/core/engine/mpeg/nv40.c

@@ -61,6 +61,7 @@ nv40_mpeg_context_ctor(struct nouveau_object *parent,
 	if (ret)
 		return ret;
 
+	nv_wo32(&chan->base.base, 0x78, 0x02001ec1);
 	return 0;
 }