i965/gen8: Don't rely directly on the hiz miptree structure

i965/gen8: Don't rely directly on the hiz miptree structure

We are still allocating a miptree for hiz, but we only use fields from
intel_miptree_aux_buffer. This will allow us to switch over to not
allocating a miptree.

Signed-off-by: Jordan Justen <jordan.l.justen@intel.com>
Reviewed-by: Topi Pohjolainen <topi.pohjolainen@intel.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Ben Widawsky <ben@bwidawsk.net>