drm/i915/mst: drop connector parameter from intel_dp_mst_compute_m_n()

intel_dp_mst_compute_m_n() doesn't need the connector. Remove the
parameter.

Reviewed-by: Imre Deak <imre.deak@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/eec2e9a2e2dc3d166ac94bb9de691246a14d3945.1735912293.git.jani.nikula@intel.com