[media] cx88: drop videobuf abuse in cx88-alsa

The alsa driver uses videobuf low-level functions that are not
available in vb2, so replace them by driver-specific functions.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>