drm: rcar-du: Drop unneeded encoder cleanup in error path

The encoder->name field can never be non-null in the error path, as that
can only be possible after a successful call to
drm_simple_encoder_init(). Drop the cleanup.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Reviewed-by: Jacopo Mondi <jacopo+renesas@jmondi.org>
Reviewed-by: Kieran Bingham <kieran.bingham+renesas@ideasonboard.com>