i965/mt: Disable aux surfaces after making miptree shareable

i965/mt: Disable aux surfaces after making miptree shareable

The entire goal of intel_miptree_make_shareable() is to permanently
disable the miptree's aux surfaces. So set
intel_mipmap_tree:disable_aux_buffers after the function's done with
discarding down the aux surfaces.

References: https://bugs.freedesktop.org/show_bug.cgi?id=98329
Reviewed-by: Topi Pohjolainen <topi.pohjolainen@intel.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Cc: Nanley Chery <nanley.g.chery@intel.com
Cc: Haixia Shi <hshi@chromium.org>
Cc: mesa-stable@lists.freedesktop.org