drm/amd/display: Update setting of DP training parameters.

[Why]
Some links are dynamically assigned link encoders on stream enablement.

[How]
Update DisplayPort training parameter determination stage that assumes
link encoder statically assigned to link.

Signed-off-by: Jimmy Kizito <Jimmy.Kizito@amd.com>
Reviewed-by: Jun Lei <Jun.Lei@amd.com>
Acked-by: Stylon Wang <stylon.wang@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>