drm/i915/display: hide global state iterators, remove unused

for_each_{new,old,oldnew}_global_obj_in_state() are only used within
intel_global_state.c, hide them there. intel_for_each_global_obj() is
unused, remove it.

Reviewed-by: Luca Coelho <luciano.coelho@intel.com>
Link: https://lore.kernel.org/r/a23212d9298423d8971d6ad62f961386f7f927cc.1753953530.git.jani.nikula@intel.com


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