drm/amd/display: Refactor vendor specific link training sequence

[Why]
Current implementation is not scalable and retrofits the existing
standard link training code for purposes outside of its original design.

[How]
Refactor vendor specific link training sequence into its own separate
function to be called instead of the standard link training function.

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