iris: assign bo->index to the aux map BOs too

I don't see these BOs being searched for in the benchmarks I tested so
I don't think this should improve anything. On the other hand, it
shouldn't hurt either since it's just an extra assignment.

I want to unify both places where we have this code into a single
function and the lack of the bo->index assignment was the only
difference between the two places. So first we make both functions the
same and in the next commit we'll unify things. This should make
bisecting easier in case I'm wrong.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/12194>

src/gallium/drivers/iris/iris_batch.c

@@ -520,6 +520,7 @@ add_aux_map_bos_to_batch(struct iris_batch *batch)
             .flags = bo->kflags,
          };
       batch->aperture_space += bo->size;
+      bo->index = batch->exec_count;
       batch->exec_count++;
    }
 }