drm: rcar-du: Enable VSP compositor by default on Gen3

On Gen3 hardware the VSP compositor is required for display. Enable it
by default in the kernel configuration. The option is kept
user-configurable for testing purpose on Gen2 platforms.

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