drm/amd/display: do not set RX back to SST mode for non 0 mst stream count

[why]
a recent regression has caused us to mistakenly switch RX back to SST mode
when there are remaining mst stream enabled to the link. We are missing
a condition check for stream count before setting RX back to SST mode.

[how]
Add stream count check condition back and do some further refactor so the
logic is easier to understand to prevent future coding error in this sequence.

Reviewed-by: Samson Tam <Samson.Tam@amd.com>
Acked-by: Qingqing Zhuo <qingqing.zhuo@amd.com>
Signed-off-by: Wenjing Liu <wenjing.liu@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>