[media] dw2102: store i2c client for tuner into dw2102_state

Prepare the dw2102 driver for tuner drivers that are implemented as I2C
drivers (such as m88ts2022). The I2C client is stored in to the state
and released at disconnect.

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