drm/i915/dp_mst: Update the total link slot count early

drm/i915/dp_mst: Update the total link slot count early

A follow up change will check a selected bpp's BW requirement in
intel_dp_mtp_tu_compute_config(), however that requires the total link
slot count to be up-to-date. The latter in turn depends on the channel
encoding and hence the link rate used, so it can be set after the
link rate used is selected.

This also allows simplifying mst_stream_update_slots(), do that as well,
moving the function definition before its use.

Cc: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Ankit Nautiyal <ankit.k.nautiyal@intel.com>
Reviewed-by: Luca Coelho <luciano.coelho@intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Link: https://lore.kernel.org/r/20250509180340.554867-5-imre.deak@intel.com