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

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

[ Upstream commit 40f43730f43699ce8557e4fe59622d4f4b69f44a ]

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

Fixes: 7a1adbd23990 ("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>
Signed-off-by: Sasha Levin <sashal@kernel.org>