drm/amd/display: implement decide lane settings

[why]
Decouple lane settings decision logic all to its own function. The
function takes in lane adjust array and link training settings and
decide what hw lane setting and dpcd lane setting should be used.

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