drm: rcar-du: Fix a NULL vs IS_ERR() bug

The drmm_encoder_alloc() function returns error pointers.  It never
returns NULL.  Fix the check accordingly.

Fixes: 7a1adbd2 ("drm: rcar-du: Use drmm_encoder_alloc() to manage encoder")
Signed-off-by: Dan Carpenter <error27@gmail.com>
Reviewed-by: Kieran Bingham <kieran.bingham+renesas@ideasonboard.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>

drivers/gpu/drm/rcar-du/rcar_du_encoder.c

@@ -109,8 +109,8 @@ int rcar_du_encoder_init(struct rcar_du_device *rcdu,
 	renc = drmm_encoder_alloc(&rcdu->ddev, struct rcar_du_encoder, base,
 				  &rcar_du_encoder_funcs, DRM_MODE_ENCODER_NONE,
 				  NULL);
-	if (!renc)
-		return -ENOMEM;
+	if (IS_ERR(renc))
+		return PTR_ERR(renc);
 
 	renc->output = output;