drm/amd/display: Fix MST topology debugfs

[why]
The drm dump_topology function was previously called on all
DP connectors. This resulted in empty topology dumps for those
connectors which weren't root MST nodes.

[how]
Make sure we only dump topology from the root MST node.

Signed-off-by: Eryk Brol <eryk.brol@amd.com>
Reviewed-by: Aurabindo Jayamohanan Pillai <Aurabindo.Pillai@amd.com>
Acked-by: Anson Jacob <Anson.Jacob@amd.com>
Tested-by: Dan Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>