anv: release memory allocated by bo_heap when descriptor pool is destroyed

anv: release memory allocated by bo_heap when descriptor pool is destroyed

Fixes following leak:

   ==21853== 32 bytes in 1 blocks are definitely lost in loss record 2 of 20
   ==21853==    at 0x483AB1A: calloc (vg_replace_malloc.c:762)
   ==21853==    by 0x4C4DD7F: util_vma_heap_free (vma.c:221)
   ==21853==    by 0x4C4D647: util_vma_heap_init (vma.c:46)
   ==21853==    by 0x4957B9F: anv_CreateDescriptorPool (anv_descriptor_set.c:578)

Fixes: c520f4dec9cb ("anv: Add a concept of a descriptor buffer")
Signed-off-by: Tapani Pälli <tapani.palli@intel.com>
Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>