drm/amd/display: Exclude MST from fake sink

Needs effort to take care of the fake sink scenario
in downstream daisy chain device. Exclude MST from
fake sink feature for now.

Signed-off-by: Jerry Zuo <Jerry.Zuo@amd.com>
Reviewed-by: Harry Wentland <Harry.Wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>