video: ipuv3: fix building with disabled panel driver

Panel code might be disabled for some boards, make this
driver code optional.

Signed-off-by: Anatolij Gustschin <agust@denx.de>
diff --git a/drivers/video/imx/mxc_ipuv3_fb.c b/drivers/video/imx/mxc_ipuv3_fb.c
index 4044473..cdef84a 100644
--- a/drivers/video/imx/mxc_ipuv3_fb.c
+++ b/drivers/video/imx/mxc_ipuv3_fb.c
@@ -645,7 +645,6 @@
 #if defined(CONFIG_DISPLAY)
 	struct udevice *disp_dev;
 #endif
-	struct udevice *panel_dev;
 	u32 fb_start, fb_end;
 	int ret;
 
@@ -672,9 +671,13 @@
 			return ret;
 	}
 #endif
-	ret = uclass_get_device(UCLASS_PANEL, 0, &panel_dev);
-	if (panel_dev)
-		panel_enable_backlight(panel_dev);
+	if (CONFIG_IS_ENABLED(PANEL)) {
+		struct udevice *panel_dev;
+
+		ret = uclass_get_device(UCLASS_PANEL, 0, &panel_dev);
+		if (panel_dev)
+			panel_enable_backlight(panel_dev);
+	}
 
 	uc_priv->xsize = gmode->xres;
 	uc_priv->ysize = gmode->yres;