drm/i915/dp_mst: Account for FEC and DSC overhead during BW allocation

Atm, the BW allocated for an MST stream doesn't take into account the
DSC control symbol (EOC) and data alignment overhead on the local (first
downstream) MST link (reflected by the data M/N/TU values) and - besides
the above overheads - the FEC symbol overhead on 8b/10b remote
(after a downstream branch device) MST links.

In addition the FEC overhead used on the local link is a fixed amount,
which only applies to certain modes, but not enough for all modes; add a
code comment clarifying this.

Fix the above by calculating the data M/N values with the total BW
overhead (not including the SSC overhead, since this isn't enabled by
the source device) and using this the PBN and TU values for the local
link and PBN for remote links (including SSC, since this is mandatory
for links after downstream branch devices).

For now keep the current fixed FEC overhead as a minimum, since this is
what bspec requires for audio functionality.

Calculate the effective link BW in a clearer way, applying the channel
coding efficiency based on the coding type. The calculation was correct
for 8b/10b, but not for 128b/132b links; this patch leaves the behavior
for this unchanged, leaving the fix for a follow-up.

v2:
- Fix TU size programmed to the HW, making it match the payload size
  programmed to the payload table.
v3:
- Add code comment about the connection between the payload's size in
  the payload table and the corresponding PBN value. (Ville)
- Add WARN_ON(remote_m_n.tu < dp_m_n.tu). (Ville)
- Add code comment about factors not accounted for by the BW
  calculation in intel_dp_mst_mode_valid_ctx() (and compute config).
  (Ville)
- Simplify calculation of PBN to remote_m_n.tu * mst_state->pbn_div.

Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Stanislav Lisovskiy <stanislav.lisovskiy@intel.com> (v1)
Signed-off-by: Imre Deak <imre.deak@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20231107001505.3370108-2-imre.deak@intel.com