drm/i915/dp_mst: Enable link training fallback for MST

drm/i915/dp_mst: Enable link training fallback for MST

Reduce the link parameters after a link training failure for MST
outputs, similarly to how this is done for SST.

For now allow the reduction only by staying in the 8b/10b vs. 128b/132b
mode. Enabling the mode switch is left for a follow-up patchset, after
taking measures ensuring that the mode switch happens properly. In
particular a rediscovery of the whole MST topology may be required for
such a switch, see the References below.

Link: https://gitlab.freedesktop.org/drm/i915/kernel/-/issues/10970
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240610164933.2947366-17-imre.deak@intel.com