[media] media: davinci: vpif_display: embed video_device struct in channel_obj

Embed video_device struct (video_dev) in channel_obj and also the
Unregister path doesn't need to free the video_device structure,
hence, change the video_device.release callback point to
video_device_release_empty.

Signed-off-by: Lad, Prabhakar <prabhakar.csengg@gmail.com>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>