drm/amd/display: Use external DP bridge encoders

drm/amd/display: Use external DP bridge encoders

Implement link creation for external DP bridge encoders
such as NUTMEG and TRAVIS used with CIK APUs such as
Kaveri for supporting analog and LVDS connections.
Typically found in CIK APU based laptops or on FM2
motherboards that have analog connectors.

When we query connector information from the VBIOS and
discover a connector using such an encoder, let's find the
real DisplayPort encoder and use that. Set the connector
signal type to DP, so the pre-existing DP code paths can
work with it without refactoring every signal type check
in the DC code base.

In the DM, make sure to report correct connector type and
also to initialize DP specifics such as the AUX channel.

Signed-off-by: Timur Kristóf <timur.kristof@gmail.com>
Reviewed-by: Alex Hung <alex.hung@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>