[media] rtl28xxu: use demod mux I2C adapter for every tuner

Tuners are connected to demod I2C adapter. Use that muxed adapter
for each tuner. That allows us to get rid of hackish FE gate control
solution.

Signed-off-by: Antti Palosaari <crope@iki.fi>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>