iris: vma_free bo->size, not bo_size

iris: vma_free bo->size, not bo_size

this is more obviously correct.  I think the two end up being the same
in practice, since this is in the alloc_from_cache case, and presumably
bo from the bucket has bo->size == bucket->size, and bo_size also is
bucket->size...

still.  better to do the obvious thing.

brw_bufmgr already does it this way.