drm/amd/display: Add DIGD case when getting retimer settings

One of the HDMI ports map to DIGD, which we did not map to DP3 ext
HDMI settings. Add the DIGD case so that we can get proper retimer
settings from SBIOS for that port.

Signed-off-by: Andrew Jiang <Andrew.Jiang@amd.com>
Reviewed-by: Charlene Liu <Charlene.Liu@amd.com>
Reviewed-by: Tony Cheng <Tony.Cheng@amd.com>
Acked-by: Harry Wentland <Harry.Wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>