video: simple-panel: Fix use of CONFIG_IS_ENABLED on DM_REGULATOR

The DM_REGULATOR symbol has SPL counterpart in:
  drivers/power/regulator/Kconfig:config SPL_DM_REGULATOR
Use CONFIG_IS_ENABLED() macro to match on the correct
variant depending on the build stage.

Signed-off-by: Marek Vasut <marek.vasut+renesas@mailbox.org>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/drivers/video/simple_panel.c b/drivers/video/simple_panel.c
index 91c91ee..092906c 100644
--- a/drivers/video/simple_panel.c
+++ b/drivers/video/simple_panel.c
@@ -53,7 +53,7 @@
 	struct simple_panel_priv *priv = dev_get_priv(dev);
 	int ret;
 
-	if (IS_ENABLED(CONFIG_DM_REGULATOR)) {
+	if (CONFIG_IS_ENABLED(DM_REGULATOR)) {
 		ret = uclass_get_device_by_phandle(UCLASS_REGULATOR, dev,
 						   "power-supply", &priv->reg);
 		if (ret) {
@@ -89,7 +89,7 @@
 	struct simple_panel_priv *priv = dev_get_priv(dev);
 	int ret;
 
-	if (IS_ENABLED(CONFIG_DM_REGULATOR) && priv->reg) {
+	if (CONFIG_IS_ENABLED(DM_REGULATOR) && priv->reg) {
 		debug("%s: Enable regulator '%s'\n", __func__, priv->reg->name);
 		ret = regulator_set_enable(priv->reg, true);
 		if (ret)