drm/amd/dm: Don't forget to attach MST encoders

The change fixed huge delay in SST daisy chain and S3 soft hang
observed in 4.19 kernel rebase.

Regression point in drm:
drm/fb-helper: Eliminate the .best_encoder() usage

The aux sequence is altered due to the failure in
drm_connector_for_each_possible_encoder(). The failure is
caused by missing attached encoder in the process of adding
MST connector.
 
drm_dp_send_enum_path_resources() aux transaction is pushed after
mode probe, which causes conflict to drm_dp_mst_i2c_xfer(),
leading to the transaction timeout.

Signed-off-by: Lyude Paul <lyude@redhat.com>
Reviewed-by: Jerry (Fangzhi) Zuo <Jerry.Zuo@amd.com>
Cc: Stable <stable@vger.kernel.org>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>