drm/sun4i: Fix error handling

'sun4i_layers_init()' returns an error pointer in case of error, not
NULL. So test it with IS_ERR.
Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>

drivers/gpu/drm/sun4i/sun4i_drv.c

@@ -142,7 +142,7 @@ static int sun4i_drv_bind(struct device *dev)
 
 	/* Create our layers */
 	drv->layers = sun4i_layers_init(drm);
-	if (!drv->layers) {
+	if (IS_ERR(drv->layers)) {
 		dev_err(drm->dev, "Couldn't create the planes\n");
 		ret = -EINVAL;
 		goto free_drm;