[Why]:
Some active dongles have DP++ port and DP port at the same time. Current
code doesn't cover DP++ case and processes as default DVI case, in which
audio is disabled. Because of dual mode, DP case is also treat as DVI case
for the other port.

[How]:
According DP 1.4 spec, add DP++ procedure similar with HDMI case. Also
add None dongle type for DP case.

Signed-off-by: Dale Zhao <dale.zhao@amd.com>
Reviewed-by: Wenjing Liu <wenjing.liu@amd.com>
Acked-by: Leo Li <sunpeng.li@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>