V4L/DVB (7452): cx88: Enable chroma AGC by default for all non-SECAM modes

An enabled chroma AGC will not degrade picture quality if enabled on a
color PAL or NTSC signal with nominal signal levels. It will give a
significant color reproduction improvement if the chroma signals
diverge from nominal levels. Therefore enable chroma AGC by default
for PAL and NTSC standards.

Signed-off-by: "Frej Drejhammar <frej.drejhammar@gmail.com>"
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>