drm/amd/display: Create debugfs to tell if connector is DPIA link

[Why]
Tests need to tell if display is connected via USB4 DPIA link.
Currently this is only possible via analyzing dmesg logs.

[How]
Create a per-connector debugfs entry to report if the link is
tunneled via USB4 DPIA.

Reviewed-by: Wayne Lin <Wayne.Lin@amd.com>
Acked-by: Jasdeep Dhillon <jdhillon@amd.com>
Signed-off-by: Stylon Wang <stylon.wang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>