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.

src/gallium/drivers/iris/iris_bufmgr.c

@@ -570,7 +570,7 @@ retry:
        * memory and assign it a new address.
        */
       if (memzone != memzone_for_address(bo->gtt_offset)) {
-         vma_free(bufmgr, bo->gtt_offset, bo_size);
+         vma_free(bufmgr, bo->gtt_offset, bo->size);
          bo->gtt_offset = 0ull;
       }
    } else {