drm/radeon/kms: Fix device tree linkage of i2c buses

Properly set the parent device of i2c buses before registering them so
that they will show at the right place in the device tree (rather than
in /sys/devices directly.)

Signed-off-by: Jean Delvare <jdelvare@suse.de>
Cc: Dave Airlie <airlied@gmail.com>
Reviewed-by: Alex Deucher <alexdeucher@gmail.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>