drm/nouveau/bios: fix oops on pre-nv50 chipsets

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

drivers/gpu/drm/nouveau/core/subdev/bios/shadowramin.c

@@ -44,8 +44,10 @@ static void
 pramin_fini(void *data)
 {
 	struct priv *priv = data;
-	nv_wr32(priv->bios, 0x001700, priv->bar0);
-	kfree(priv);
+	if (priv) {
+		nv_wr32(priv->bios, 0x001700, priv->bar0);
+		kfree(priv);
+	}
 }
 
 static void *