drm: rcar-du: Depend on DRM_RCAR_DU for components on that SoC

There's a few components in the rcar-du drm directory that don't make
sense to be selectable if DRM_RCAR_DU isn't because they are part of the
IP block so add a dependency. The drivers are still testable with
COMPILE_TEST on non-Renesas platforms as DRM_RCAR_DU itself is
selectable with COMPILE_TEST.

Signed-off-by: Peter Robinson <pbrobinson@gmail.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>