drm/sun4i: rgb: panel is an error pointer

In case of an error, our pointer to the drm_panel structure attached to our
encoder will hold an error pointer, not a NULL pointer.

Make sure we check the right thing.

Fixes: 29e57fab97 ("drm: sun4i: Add RGB output")
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>

drivers/gpu/drm/sun4i/sun4i_rgb.c

@@ -217,7 +217,7 @@ int sun4i_rgb_init(struct drm_device *drm)
 	int ret;
 
 	/* If we don't have a panel, there's no point in going on */
-	if (!tcon->panel)
+	if (IS_ERR(tcon->panel))
 		return -ENODEV;
 
 	rgb = devm_kzalloc(drm->dev, sizeof(*rgb), GFP_KERNEL);