drm/amd/display: Save/restore link setting for disable phy when link retraining

[Why]
The link setting will be modify after disable phy
and due to DP Compliance Fails.

[How]
Save and resotre link setting for disable link phy when link retraining.

Signed-off-by: Hugo Hu <hugo.hu@amd.com>
Reviewed-by: Wenjing Liu <Wenjing.Liu@amd.com>
Acked-by: Leo Li <sunpeng.li@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>