[media] v4l2: add device_caps to struct video_device

Instead of letting drivers fill in device_caps at querycap time,
let them fill it in when the video device is registered.

This has the advantage that in the future the v4l2 core can access
the video device's capabilities and take decisions based on that.

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