drm/i915/dp_mst: Enable LT fallback between UHBR/non-UHBR link rates

Enable switching between UHBR and non-UHBR link rates on MST links when
reducing the link parameters after an LT failure.

Reviewed-by: Suraj Kandpal <suraj.kandpal@intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240722165503.2084999-15-imre.deak@intel.com