drm: rcar-du: Make DRM_RCAR_WRITEBACK depends on DRM_RCAR_DU

drm: rcar-du: Make DRM_RCAR_WRITEBACK depends on DRM_RCAR_DU

There is no need to select DRM_RCAR_WRITEBACK if DRM_RCAR_DU=n which
just make the generated .config a bit ugly.

 # ARM devices
 #
 # end of ARM devices

 CONFIG_DRM_RCAR_WRITEBACK=y

 #
 # Frame buffer Devices

Let DRM_RCAR_WRITEBACK depend on DRM_RCAR_DU instead.

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