drm/panel: panel-simple: Add dev_err_probe if backlight could not be found

If the backlight node is not enabled, this (silently) returns with
-EPROBE_DEFER. /sys/kernel/debug/devices_deferred also shows nothing
helpful:
$ cat /sys/kernel/debug/devices_deferred
display

With this patch, there is a helpful hint:
$ cat /sys/kernel/debug/devices_deferred
display panel-simple: Could not find backlight
Signed-off-by: Alexander Stein <alexander.stein@ew.tq-group.com>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20220621072118.513346-1-alexander.stein@ew.tq-group.com

drivers/gpu/drm/panel/panel-simple.c

@@ -675,8 +675,10 @@ static int panel_simple_probe(struct device *dev, const struct panel_desc *desc)
 	drm_panel_init(&panel->base, dev, &panel_simple_funcs, connector_type);
 
 	err = drm_panel_of_backlight(&panel->base);
-	if (err)
+	if (err) {
+		dev_err_probe(dev, err, "Could not find backlight\n");
 		goto disable_pm_runtime;
+	}
 
 	drm_panel_add(&panel->base);