drm/amd/display: Skip SST ACT polling when sink_count is 0

[Why]
Upon disconnecting a UHBR SST display, the disconnection and the
SetTimings to disable the display can occur such that link_set_dpms_off
occurs after the disconnection has already processed
(link->type is dc_connection_none). Thus, the AUX related
operations should be skipped when clearing the payload allocation table.

Reviewed-by: Wenjing Liu <wenjing.liu@amd.com>
Acked-by: Aurabindo Pillai <aurabindo.pillai@amd.com>
Signed-off-by: George Shen <george.shen@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>