drm/amd/display: determine is mst hdcp based on stream instead of sink signal

[why]
It is possible even if sink signal is MST but driver enables SST stream.
We should not determine if we should do MST authentication based on
sink's capability.
Instead we should determine whether to do MST authentication based on
what we have enabled in stream.

Signed-off-by: Wenjing Liu <Wenjing.Liu@amd.com>
Reviewed-by: Ashley Thomas <Ashley.Thomas2@amd.com>
Acked-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>