drm/panel: lvds: Handle the optional regulator case properly

The devm_regulator_get_optional function, unlike it was assumed in the
commit a1c55bcc ("drm/panel: lvds: Add support for the power-supply
property"), is actually returning an error pointer with -ENODEV instead of
NULL when there's no regulator to find.

Make sure we handle that case properly.
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Tested-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Fixes: a1c55bcc ("drm/panel: lvds: Add support for the power-supply property")
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180110155941.16109-1-maxime.ripard@free-electrons.com

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

@@ -215,10 +215,17 @@ static int panel_lvds_probe(struct platform_device *pdev)
 	lvds->supply = devm_regulator_get_optional(lvds->dev, "power");
 	if (IS_ERR(lvds->supply)) {
 		ret = PTR_ERR(lvds->supply);
-		dev_err(lvds->dev, "failed to request regulator: %d\n", ret);
+
+		if (ret != -ENODEV) {
+			if (ret != -EPROBE_DEFER)
+				dev_err(lvds->dev, "failed to request regulator: %d\n",
+					ret);
 			return ret;
 		}
 
+		lvds->supply = NULL;
+	}
+
 	/* Get GPIOs and backlight controller. */
 	lvds->enable_gpio = devm_gpiod_get_optional(lvds->dev, "enable",
 						     GPIOD_OUT_LOW);