drm: rcar-du: Drop local encoder variable

The local encoder variable is an alias for &renc->base, and is only use
twice. It doesn't help much, drop it, along with the
rcar_encoder_to_drm_encoder() macro that is then unused.

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>