drm: rcar-du: Rely on the default ->best_encoder() behavior

All outputs have a 1:1 relationship between connectors and encoders,
and the driver is relying on the atomic helpers: we can drop the custom
->best_encoder() implementations and let the core call
drm_atomic_helper_best_encoder() for us.

Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1465300095-16971-10-git-send-email-boris.brezillon@free-electrons.com