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

src/mesa/drivers/dri/i965/intel_mipmap_tree.c

@@ -2330,6 +2330,8 @@ intel_miptree_make_shareable(struct brw_context *brw,
       free(mt->mcs_buf);
       mt->mcs_buf = NULL;
    }
+
+   mt->disable_aux_buffers = true;
 }