[media] saa7134: fix format-related compliance issues

- map overlay format values to the supported ranges
- set colorspace
- zero priv field
- fix cliplist handling
- fix field handling
- initialize ovbuf values

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