drm/amd/display: MST DSC check for older devices

[Why]
Some older MST hubs do not report DPCD registers according to
specification.

[How]
This change re-applies commit c5365554

 ("drm/amd/display: dsc mst
re-compute pbn for changes on hub").
With an additional check for these older MST devices.

Reviewed-by: Swapnil Patel <swapnil.patel@amd.com>
Acked-by: Tom Chung <chiahsuan.chung@amd.com>
Signed-off-by: Agustin Gutierrez <agustin.gutierrez@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>