drm: rcar-du: Make sure the VSP is initialized on platforms that need it

On Gen3 platforms planes are managed by the external VSP compositor on
behalf of DRM/KMS. If VSP compositor support is not enabled in the DU
driver, the VSP initialization stub routine is called. Return an error
from that stub to fail explicitly, otherwise the device won't be usable
and the driver will crash.

Signed-off-by: Jacopo Mondi <jacopo+renesas@jmondi.org>
Reviewed-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
[Clarified commit message]
Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>