i965/hiz: Start to separate miptree out from hiz buffers

i965/hiz: Start to separate miptree out from hiz buffers

Today we allocate a miptree's for the hiz buffer. We needed this in
the past because we would point the hardware at offsets of the hiz
buffer. Since the hiz format is not documented, this is not a good
idea.

Since moving to support layered rendering on Gen7+, we no longer point
at an offset into the buffer on Gen7+.

Therefore, to support hiz on Gen7+, we don't need a full miptree
structure allocated.

This patch starts to create a new auxiliary buffer structure
(intel_miptree_aux_buffer) that can be a more simplistic miptree
side-band buffer associated with a miptree. (For example, to serve the
needs of the hiz buffer.)

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>