drm/amd/display: Implement DPIA equalisation phase

[Why]
Equalisation is the mandatory second phase of DisplayPort link training
over a USB4 DP tunnel.

[How]
Implement equalisation phase for DP tunneled over USB4 in DPIA
training module.

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>