drm/radeon/kms: connector fixes

- Don't add dac load detection property to DVI-D
- Make sure i2c info is valid before adding DP aux chan bus
- Don't create scaling_mode_property twice
- fix typo that prevented coherent and load detection from working
- add coherent prop to DP (for dp->dvi adapters)

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