drm/amd/display: Add ability to set preferred link training parameters.

[WHY]
To add support for OS requirement to set preferred link training
parameters.

[HOW]
Create new structure of dp link training overrides. During link training
processes, these values should be used instead of the default training
parameters.

Signed-off-by: David Galiffi <David.Galiffi@amd.com>
Reviewed-by: Tony Cheng <Tony.Cheng@amd.com>
Acked-by: Anthony Koo <Anthony.Koo@amd.com>
Acked-by: Leo Li <sunpeng.li@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>