drm/i915/mst: update slot information for 128b/132b

128b/132b supports using 64 slots starting from 0, while 8b/10b reserves
slot 0 for metadata.

Commit d6c6a76f ("drm: Update MST First Link Slot Information Based
on Encoding Format") added support for updating the topology state
accordingly, and commit 41724ea2

 ("drm/amd/display: Add DP 2.0 MST
DM Support") started using it in the amd driver.

This feels more than a little cumbersome, especially updating the
information in atomic check. For i915, add the update to MST connector
.compute_config hook rather than iterating over all MST managers and
connectors in global mode config .atomic_check. Fingers crossed.

v3:
- Propagate errors from intel_dp_mst_update_slots() (Ville)

v2:
- Update in .compute_config() not .atomic_check (Ville)

Cc: Bhawanpreet Lakha <Bhawanpreet.Lakha@amd.com>
Cc: Lyude Paul <lyude@redhat.com>
Cc: Uma Shankar <uma.shankar@intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Acked-by: Lyude Paul <lyude@redhat.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220208152317.3019070-1-jani.nikula@intel.com