[media] af9015: limit I2C access to keep FW happy

AF9015 firmware does not like if it gets interrupted by I2C adapter
request on some critical phases. During normal operation I2C adapter
is used only 2nd demodulator and tuner on dual tuner devices.

Override demodulator callbacks and use mutex for limit access to
those "critical" paths to keep AF9015 happy.

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