drm/amd/display: Add debugfs initalization on mst connectors

[why]
We were missing debugfs files on MST connectors as the files
weren't initialized.

[how]
Move connector debugfs initialization into connoctor's
init helper function so it will be called by both SST and MST
connectors. Also move connector registration so it will be
registered before we create the entries.

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>