[media] media: am437x-vpfe: embed video_device struct in vpfe_device

Embed video_device struct (video_dev) in vpfe_device and
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>