drm/radeon: only add the mm i2c bus if the hw_i2c module param is set

It seems it can corrupt the monitor EDID in certain cases on certain
boards when running sensors detect.  It's rarely used anyway outside
of AIW boards.

http://lists.lm-sensors.org/pipermail/lm-sensors/2012-April/035847.html
http://lists.freedesktop.org/archives/xorg/2011-January/052239.html

Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org
Acked-by: Jean Delvare <khali@linux-fr.org>
Signed-off-by: Dave Airlie <airlied@redhat.com>