drm/i915/dp: constify struct link_config_limits pointers

The limits get passed around, but are only modified in a few
places. Constify the pointers elsewhere so it's easier to follow where
they can be modified.

Reviewed-by: Imre Deak <imre.deak@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/6ab2f68eef7849aca18e82ad788e44e9f82b576e.1738161945.git.jani.nikula@intel.com


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