drm/i915/mst: split out a helper for figuring out the TU

Extract intel_dp_mtp_tu_compute_config() for figuring out the TU. Move
the link configuration and mst state access to the callers. This will be
easier to adapt to 128b/132b SST.

v2: Don't add SST stuff here yet

Reviewed-by: Imre Deak <imre.deak@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/c3ea8370c9bd3cdc579159e68a63f4ed2fadc66a.1735912293.git.jani.nikula@intel.com