drm/amd/display: isolate link training setting override to its own function

There is a difference between our default behavior and override
behavior. For default behavior we need to decide link training settings
within specs' limitation and mandates.
For override behavior we do not need to follow all these requirements.
We are isolating override decision to its own function to maintain the
integrity of our specs compliant default behavior.

Signed-off-by: Wenjing Liu <wenjing.liu@amd.com>
Reviewed-by: George Shen <George.Shen@amd.com>
Acked-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>