drm/amd/display: Fix dc_sink kref count in emulated_link_detect

[why]
prev_sink is not used anywhere else in the function and the reference to
it from dc_link is replaced with a new dc_sink.

[how]
Change dc_sink_retain(prev_sink) to dc_sink_release(prev_sink).

Signed-off-by: Victor Lu <victorchengchi.lu@amd.com>
Reviewed-by: Nicholas Kazlauskas <Nicholas.Kazlauskas@amd.com>
Acked-by: Anson Jacob <Anson.Jacob@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>