media: vivid: add dynamic array test control

Add a dynamic array test control to help test support for this
feature.

Acked-by: Nicolas Dufresne <nicolas.dufresne@collabora.com>
Tested-by: Benjamin Gaignard <benjamin.gaignard@collabora.com>
Tested-by: Jernej Skrabec <jernej.skrabec@gmail.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>