drm/amd/display: Remove reliance on pipe indexing

[Why]
In certain instances, there was a reliance on pipe indexing being accurate. However, this
assumption fails with harvesting of pipes 1 or 2, which can occur in production B6 parts.
HW hang would occur as a result.

[How]
Use hubp index for mpcc, and do mpc_init for all theoretical pipes (including disabled ones).

Signed-off-by: Noah Abradjian <noah.abradjian@amd.com>
Reviewed-by: Yongqiang Sun <yongqiang.sun@amd.com>
Acked-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>