drm/amd/display: Fix debugfs on MST connectors

[why]
Previous patch allowed to initialize debugfs entries on both MST
and SST connectors, but MST connectors get registered much later
which exposed an issue of debugfs entries being initialized in the
same folder.

[how]
Return SST debugfs entries' initialization back to where it was.
For MST connectors we should initialize debugfs entries in connector
register function after the connector is registered.

Signed-off-by: Mikita Lipski <mikita.lipski@amd.com>
Reviewed-by: Nicholas Kazlauskas <Nicholas.Kazlauskas@amd.com>
Acked-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>