drm: rcar-du: writeback: include interface header

The new writeback feature exports functions so that they can integrate
into the rcar_du_kms module.

The interface functions are defined in the rcar_du_writeback header, but
it is not included in the object file itself leading to compiler
warnings for missing prototypes.

Include the header as appropriate.

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