drm/exynos: checking for NULL instead of IS_ERR()

The of_drm_find_panel() function returns error pointers and never NULL
but we the driver assumes that ->panel is NULL when it's not present.

Fixes: 6afb7721e2a0 ("drm/exynos: move connector creation to attach callback")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>

drivers/gpu/drm/exynos/exynos_drm_dsi.c

@@ -1527,7 +1527,9 @@ static int exynos_dsi_host_attach(struct mipi_dsi_host *host,
 		}
 
 		dsi->panel = of_drm_find_panel(device->dev.of_node);
-		if (dsi->panel) {
+		if (IS_ERR(dsi->panel)) {
+			dsi->panel = NULL;
+		} else {
 			drm_panel_attach(dsi->panel, &dsi->connector);
 			dsi->connector.status = connector_status_connected;
 		}