[media] v4l: vsp1: Make the userspace API optional

The R-Car Gen3 SoCs include VSP instances dedicated to the DU that will
be controlled entirely by the rcar-du-drm driver through the KMS API. To
support that use case make the userspace V4L2 API optional.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>