drm/amd/display: VGA black screen from s3 when attached to hook

[Description] For MST, DC already notify MST sink for MST mode, DC stll
check DP SINK DPCD register to see if MST enabled. DP RX firmware may
not handle this properly.

Signed-off-by: Hersen Wu <hersenxs.wu@amd.com>
Reviewed-by: Tony Cheng <Tony.Cheng@amd.com>
Acked-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>