drm/amd/display: Add audio support for DFP type of active branch is DP case

[Why]
Per DP spec, for active protocol convertor adaptor, DP source should enable audio
for DFP type is DP, HDMI or DP++. Current is_dp_active_dongle() checking is not
precise, which treat branch device default as active dongle. As a result, we will
mistakenly disable audio for DFP type is DP case.

[How]
Make is_dp_active_dongle() checking more precise for active dongle types.
Rename active diongle type as SST branch device in case confusion.

Signed-off-by: Dale Zhao <dale.zhao@amd.com>
Reviewed-by: Wenjing Liu <Wenjing.Liu@amd.com>
Acked-by: Wayne Lin <Wayne.Lin@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>