[media] v4l: subdev: Add new file operations

V4L2 sub-devices store pad formats and crop settings in the file handle.
To let drivers initialize those settings properly, add an open operation
that is called when the subdev is opened as well as a corresponding
close operation.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Acked-by: Hans Verkuil <hverkuil@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>