drm/amd/display: update DSC MST DP virtual DPCD peer device enumeration policy

[why]
Current policy assumes virtual DPCD peer device as
an individual MST branch device with 1 input and 1 output.
However this is only true for virtual DP-to-DP peer device.
In general there are three types of virtual DP peer devices.
1. Sink peer device with virtual DPCD.
2. Virtual DP-to-DP Peer device with virtual DPCD.
3. Virtual DP-to-HDMI Protocol Converter Peer Device with
Virtual DPCD.
So we should break the assumption and handle all three types.

[how]
DP-to-DP peer device will have virtual DPCD cap upstream.
Sink peer device will have virtual DPCD on the logical port.
Dp to HDMI protocol converter peer device will have virtual DPCD
on its converter port.
For DSC capable Synaptics non VGA port we workaround by enumerating
a virutal DPCD peer device on its upstream
even if it doesn't have one.

Signed-off-by: Wenjing Liu <Wenjing.Liu@amd.com>
Reviewed-by: Jun Lei <Jun.Lei@amd.com>
Acked-by: Bhawanpreet Lakha <Bhawanpreet.Lakha@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>