[media] dvb-frontends/stv0367: add flag to make i2c_gatectrl optional

Some hardware and bridges (namely ddbridge) require that tuner access is
limited to one concurrent access and wrap i2c gate control with a
mutex_lock when attaching frontends. According to vendor information, this
is required as concurrent tuner reconfiguration can interfere each other
and at worst cause tuning fails or bad reception quality.

If the demod driver does gate_ctrl before setting up tuner parameters, and
the tuner does another I2C enable, it will deadlock forever when gate_ctrl
is wrapped into the mutex_lock. This adds a flag and a conditional before
triggering gate_ctrl in the demodulator driver.

Signed-off-by: Daniel Scheller <d.scheller@gmx.net>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>