drm/amd/display: dsc mst re-compute pbn for changes on hub

[why]
when unplug 1 dp from dsc mst hub, atomic_check new request
dc_state only include info for the unplug dp. this will not
trigger re-compute pbn for displays still connected to hub.

[how] all displays connected to dsc hub are available in
dc->current_state, by comparing dc->current_state and new
request from atomic_chceck, it will provide info of
displays connected to hub and do pbn re-compute.

Reviewed-by: Roman Li <Roman.Li@amd.com>
Acked-by: Jasdeep Dhillon <jdhillon@amd.com>
Signed-off-by: Hersen Wu <hersenwu@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>