drm/panel: ili9341: fix optional regulator handling

If the optional regulator lookup fails, reset the pointer to NULL.
Other functions such as mipi_dbi_poweron_reset_conditional() only do
a NULL pointer check and will otherwise dereference the error pointer.

Fixes: 5a042273 ("drm/panel: Add ilitek ili9341 panel driver")
Signed-off-by: Daniel Mack <daniel@zonque.org>
Cc: stable@vger.kernel.org
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20220317225537.826302-1-daniel@zonque.org

drivers/gpu/drm/panel/panel-ilitek-ili9341.c

@@ -612,8 +612,10 @@ static int ili9341_dbi_probe(struct spi_device *spi, struct gpio_desc *dc,
 	int ret;
 
 	vcc = devm_regulator_get_optional(dev, "vcc");
-	if (IS_ERR(vcc))
+	if (IS_ERR(vcc)) {
 		dev_err(dev, "get optional vcc failed\n");
+		vcc = NULL;
+	}
 
 	dbidev = devm_drm_dev_alloc(dev, &ili9341_dbi_driver,
 				    struct mipi_dbi_dev, drm);