drm/amd/display: Update DP link configuration.

drm/amd/display: Update DP link configuration.

[Why & How]
- Update application of training settings for links whose encoders are
assigned dynamically.
- Add functionality useful for DP link configuration to public
interface.

Signed-off-by: Jimmy Kizito <Jimmy.Kizito@amd.com>
Reviewed-by: Jun Lei <Jun.Lei@amd.com>
Acked-by: Qingqing Zhuo <qingqing.zhuo@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>