[media] m88ds3103: remove unneeded AGC from inittab

Optimal AGC is highly depended on used RF tuner and due to that
it is already included to chip configuration. However, inittab
has default AGC value, which was later replaced by one from config.

Add also comment to all chip configuration options about default
values and if those are needed to set or not.

Reported-by: David Howells <dhowells@redhat.com>
Signed-off-by: Antti Palosaari <crope@iki.fi>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>