drm/rcar-du: Add run-time dependencies

The Renesas R-Car Display Unit driver is only useful on shmobile
unless build testing. The LVDS output is useful on an even more
reduced hardware set.

Signed-off-by: Jean Delvare <jdelvare@suse.de>
Acked-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>