ivtv_yuv_prep_frame is split in smaller code blocks.

Modified yuv buffer handling on the PVR350 itself. We now cycle through all 8
hardware buffers.

With this patch in place, driver behaviour should remain unchanged from the
existing release.
Signed-off-by: Ian Armstrong <ian@iarmst.demon.co.uk>
Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>