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

Properly set the parent device of DP i2c buses before registering them
too.

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