drm/amd/display: Add fallback and abort paths for DP link training.

[Why]
When enabling a DisplayPort stream:
- Optionally reducing link bandwidth between failed link training
attempts should progressively relax training requirements.
- Abandoning link training altogether if a sink is unplugged should
avoid unnecessary training attempts.

[How]
- Add fallback parameter to DP link training function and reduce link
bandwidth between failed training attempts as long as stream bandwidth
requirements are met.
- Add training status for sink unplug and abort training when this
status is reported.

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>