drm/gf100-/gr: fix -ENOSPC detection when allocating zbc table entries

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

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

@@ -68,6 +68,9 @@ nvc0_graph_zbc_color_get(struct nvc0_graph_priv *priv, int format,
 		}
 	}
 
+	if (zbc < 0)
+		return zbc;
+
 	memcpy(priv->zbc_color[zbc].ds, ds, sizeof(priv->zbc_color[zbc].ds));
 	memcpy(priv->zbc_color[zbc].l2, l2, sizeof(priv->zbc_color[zbc].l2));
 	priv->zbc_color[zbc].format = format;
@@ -109,6 +112,9 @@ nvc0_graph_zbc_depth_get(struct nvc0_graph_priv *priv, int format,
 		}
 	}
 
+	if (zbc < 0)
+		return zbc;
+
 	priv->zbc_depth[zbc].format = format;
 	priv->zbc_depth[zbc].ds = ds;
 	priv->zbc_depth[zbc].l2 = l2;