ARM: shmobile: lager: Select DRM_RCAR_DU in defconfig

A R-Car DU DRM device is registered for the Lager board, select the
corresponding driver in its defconfig.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Acked-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>