drm: rcar-du: Replace direct DRM encoder access with cast macro

Add a new macro to downcast an rcar_du_encoder pointer to a drm_encoder
pointer and use it. This prepares for the replacement of the
rcar_drm_encoder encoder field with a drm_slave_encoder.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>