drm/amd/display: Release MST resources on switch from MST to SST

[why]
When OS overrides training link training parameters
for MST device to SST mode, MST resources are not
released and leak of the resource may result crash and
incorrect MST discovery during following hot plugs.

[how]
Retaining sink object to be reused by SST link and
releasing MST  resources.

Signed-off-by: Vladimir Stempen <vladimir.stempen@amd.com>
Reviewed-by: Wenjing Liu <Wenjing.Liu@amd.com>
Acked-by: Stylon Wang <stylon.wang@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>