[media] si2165: only write agc registers after reset before start_syncro

Datasheet says they must be rewritten after reset.
But it only makes sense to write them when trying to tune afterwards.

Signed-off-by: Matthias Schwarzott <zzam@gentoo.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>