drm/amd/display: Implement DPIA training loop

[Why]
Training of DPIA link differs enough from that of conventional
DP link to warrant a separate implementation.

[How]
- Implement top-level of DPIA training loop.
- Make functions shared between DP and DPIA link training "public".

Reviewed-by: Jun Lei <Jun.Lei@amd.com>
Acked-by: Wayne Lin <Wayne.Lin@amd.com>
Acked-by: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Acked-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Jimmy Kizito <Jimmy.Kizito@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>