drm/gf100-/gr: return non-fatal error code when fw not present

This allows the module to load without acceleration.
Signed-off-by: NBen Skeggs <bskeggs@redhat.com>

drivers/gpu/drm/nouveau/core/engine/graph/nvc0.c

@@ -1557,7 +1557,7 @@ nvc0_graph_ctor(struct nouveau_object *parent, struct nouveau_object *engine,
 		    nvc0_graph_ctor_fw(priv, "fuc409d", &priv->fuc409d) ||
 		    nvc0_graph_ctor_fw(priv, "fuc41ac", &priv->fuc41ac) ||
 		    nvc0_graph_ctor_fw(priv, "fuc41ad", &priv->fuc41ad))
-			return -EINVAL;
+			return -ENODEV;
 		priv->firmware = true;
 	}