V4L/DVB: v4l videobuf: move video_copy_to_user and copy_stream to core

The video_copy_to_user and copy_stream ops are almost identical for all
videobuf memtype variants. All that is needed is to use the new vaddr
op and these functions can be moved into the core, ensuring we have just
one single implementation instead of three.

Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>