drm/i915/dp_mst: Improve BW sharing between MST streams

drm/i915/dp_mst: Improve BW sharing between MST streams

At the moment modesetting a stream CRTC will fail if the stream's BW
along with the current BW of all the other streams on the same MST link
is above the total BW of the MST link. Make the BW sharing more dynamic
by trying to reduce the link bpp of one or more streams on the MST link
in this case.

When selecting a stream to reduce the BW for, take into account which
link segment in the MST topology ran out of BW and which streams go
through this link segment. For instance with A,B,C streams in the same
MST topology A and B may share the BW of a link segment downstream of a
branch device, stream C not downstream of the branch device, hence not
affecting this BW. If this link segment's BW runs out one or both of
stream A/B's BW will be reduced until their total BW is within limits.

While reducing the link bpp for a given stream DSC may need to be
enabled for it, which requires FEC on the whole MST link. Check for this
condition and recompute the state for all streams taking the FEC
overhead into account (on 8b/10b links).

v2:
- Rebase on s/min_bpp_pipes/min_bpp_reached_pipes/ change.

Reviewed-by: Stanislav Lisovskiy <stanislav.lisovskiy@intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20231030155843.2251023-29-imre.deak@intel.com