drm/i915/mst: use min_array() and max_array() instead of hand-rolling

Improve code clarity by using existing min_array() and max_array()
helpers to find the lowest and highest values in an array.

Reviewed-by: Imre Deak <imre.deak@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/62a104535c01c667a99ec209c3218a13355568cf.1738161945.git.jani.nikula@intel.com


Signed-off-by: Jani Nikula <jani.nikula@intel.com>