[media] cx18: Clean up mmap() support for raw YUV

The initial version of this patch (commit
d5976931639176bb6777755d96b9f8d959f79e9e) had some issues:

 * It didn't correctly calculate the size of the YUV buffer for 4:2:2,
   resulting in capture sometimes being offset by 1/3rd of a picture.

 * There were a lot of variables duplicating information the driver
   already knew, which have been removed.

 * There was an in-kernel format conversion - libv4l can do this one,
   and is the right place to do format conversions anyway.

 * Some magic numbers weren't properly explained.

Fix all these issues, leaving just the move from videobuf to videobuf2
to do.

Signed-off-by: Simon Farnsworth <simon.farnsworth@onelan.co.uk>
Acked-by: Andy Walls <awalls@md.metrocast.net>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>