drm/amd/display: use link type to decide stream enc acquisition

[Why]
Virtual sink is used when set mode happens on a disconnected display
to allow the mode set to proceed. This did not work with MST because
the logic for acquiring stream encoder uses stream signal to determine
the special handling is required, and stream signal is virtual instead
of DP in this case.

[How]
Use link type to decide instead.

Signed-off-by: Eric Yang <Eric.Yang2@amd.com>
Reviewed-by: Tony Cheng <Tony.Cheng@amd.com>
Acked-by: Bhawanpreet Lakha <Bhawanpreet.Lakha@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>