i965/miptree: Take an aux_usage in prepare/finish_render

Both callers of intel_miptree_prepare/finish_render have to call
intel_miptree_render_aux_usage anyway for other reasons.  They may as
well pass the result in instead of us calling it again.

Cc: mesa-stable@lists.freedesktop.org
Reviewed-by: Iago Toral Quiroga <itoral@igalia.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
(cherry picked from commit dfe02179)