drm/amd/display: Add debugfs for connector's FEC & DSC capabilities

[why & how]
Useful entry to understand if link has DSC or FEC capabilities,
implemented to read DPCD caps stored on the link. Better than
manually reading the registers with aux dpcd helper.

Signed-off-by: Eryk Brol <eryk.brol@amd.com>
Signed-off-by: Mikita Lipski <mikita.lipski@amd.com>
Reviewed-by: Mikita Lipski <Mikita.Lipski@amd.com>
Acked-by: Eryk Brol <eryk.brol@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>