[media] drxd: allow functional gate control after, attach

Previously, gate control didn't work until drxd_init()
execution. Migrate necessary set of commands in drxd_attach
to allow gate control to be used by tuner which are
accessible through i2c gate.

Reported-by: frederic.mantegazza@gbiloba.org
Signed-off-by: Patrice Chotard <patricechotard@free.fr>
Reviewed-by: Antti Palosaari <crope@iki.fi>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>