drm/amd/display: Add is_mst_connector debugfs entry

[Why & How]
Add "is_mst_connector" debugfs entry to help distinguish whether
a connector is in a mst topology or not.

Access it with the following command:
    cat /sys/kernel/debug/dri/0/DP-X/is_mst_connector

Result:
- "root" stands for the root connector of the topology
- "branch" stands for branch device of the topology
- "end" stands for leaf node connector of the topology
- "no" stands for the connector is not a device of a mst topology

Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Reviewed-by: Hersen Wu <hersenxs.wu@amd.com>
Acked-by: Alan Liu <HaoPing.Liu@amd.com>
Signed-off-by: Wayne Lin <Wayne.Lin@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>