drm/radeon/dp: Probe branch/sink OUIs

Signed-off-by: Adam Jackson <ajax@redhat.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Jerome Glisse <jglisse@redhat.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>