ilo: convert GPE GEN6 command functions to use ilo_builder

Similar to the changes to GEN7 command functions, but to GEN6 this time.
ilo: remove unused ilo_cp functions

Remove

  ilo_cp_begin()
  ilo_cp_steal()
  ilo_cp_write()
  ilo_cp_write_multi()
  ilo_cp_write_bo()
  ilo_cp_end()
  ilo_cp_steal_ptr()