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>