drm: rcar-du: Remove test for impossible error condition

The driver has lost platform data support a long time ago. R-Car DU
devices can only be instantiated through DT now, making it impossible to
have a NULL DT node pointer. Remove the error check.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Reviewed-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk>