drm/radeon: fix hotplug of DP to DVI|HDMI passive adapters (v2)

No need to retrain the link for passive adapters.

v2: agd5f
- no passive DP to VGA adapters, update comments
- assign radeon_connector_atom_dig after we are sure
  we have a digital connector as analog connectors
  have different private data.
- get new sink type before checking for retrain.  No
  need to check if it's no longer a DP connection.

Cc: <stable@vger.kernel.org>
Signed-off-by: Jerome Glisse <jglisse@redhat.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>