drm/radeon/kms: add hw_i2c module option

Turn off hw i2c by default except for mm i2c which
is hw only until we sort out the remaining prescale
issues on older chips.  hw i2c can be enabled with
hw_i2c=1.

Signed-off-by: Alex Deucher <alexdeucher@gmail.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>