V4L/DVB (6056): ivtv: move serialization to the fileops level

Serialization is now done on the open/close/ioctl level and also when the
read/write/poll start an encoder/decoder stream.

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