[media] cx18: mmap() support for raw YUV video capture

Add support for mmap method streaming of raw YUV video on cx18-based
hardware, in addition to the existing support for read() streaming of
raw YUV and MPEG-2 encoded video.

[simon.farnsworth@onelan.co.uk: I forward-ported this from Steven's original work,
 done under contract to ONELAN. The original code is at
 http://www.kernellabs.com/hg/~stoth/cx18-videobuf

]

Signed-off-by: Steven Toth <stoth@kernellabs.com>
Signed-off-by: Simon Farnsworth <simon.farnsworth@onelan.co.uk>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>