[media] drxd: provide ability to disable the i2c gate control function

If the tuner is not actually behind an i2c gate, using the i2c gate control
function can wedge the i2c bus.  Provide the ability to control on a per-board
basis whether it should be used.

Problem was noticed on the HVR-900 R2, where it resulted in the first tuning
attempt succeeding, and then all subsequent attempts to access the xc3028
being treated as failures (including the call to sleep the tuner).

Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>