drm/amd/display: fix graphics hang in multi-display mst case

[what]
Graphics hang observed with 3 displays connected to DP2.0 mst dock.

[why]
There's a mismatch in dml and dc between the assignments of hpo link
encoders.

[how]
Add a new array in dml that tracks the current mapping of HPO stream
encoders to HPO link encoders in dc.

Reviewed-by: Sung joon Kim <sungjoon.kim@amd.com>
Reviewed-by: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Signed-off-by: Gabe Teeger <Gabe.Teeger@amd.com>
Signed-off-by: Hamza Mahfooz <hamza.mahfooz@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>