drm/nve4/graph: do not crash if no power device present

Detect and workaround the absence of a power device so chips that do not
feature one (e.g. GK20A) can still use this driver.

Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>

drivers/gpu/drm/nouveau/core/engine/graph/nve4.c

@@ -203,7 +203,8 @@ nve4_graph_init(struct nouveau_object *object)
 	int gpc, tpc, rop;
 	int ret, i;
 
-	ppwr->pgob(ppwr, false);
+	if (ppwr)
+		ppwr->pgob(ppwr, false);
 
 	ret = nouveau_graph_init(&priv->base);
 	if (ret)