drm/amd/display: Fix for tile MST

- Set stream signal type to be SST when setting non-tile timing on MST
  tiled display.
  - Disable MST on sink after disabling MST link.
  - Enable MST on sink before enabling MST link.

Signed-off-by: Ding Wang <Ding.Wang@amd.com>
Acked-by: Harry Wentland <Harry.Wentland@amd.com>
Reviewed-by: Jun Lei <Jun.Lei@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>