drm/amd/display: Refactor fixed VS logic for non-transparent mode

[Why]
All fixed VS/PE link training sequence should be refactored
into a separate function outside of the standard link training
sequence. This includes the sequence for non-transparent
mode.

[How]
Isolate link training sequence for fixed VS/PE non-transparent
mode into a separate function.

Reviewed-by: Wenjing Liu <Wenjing.Liu@amd.com>
Acked-by: Solomon Chiu <solomon.chiu@amd.com>
Signed-off-by: George Shen <George.Shen@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>