[media] cx25821: g/s/try/enum_fmt related fixes and cleanups

- fill in colorspace
- zero priv
- delete unsupported formats
- fix field handling
- s_std should update width/height
- proper mapping of width/height to valid resolutions

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