i965: Replace draw_aux_buffer_disabled with draw_aux_usage

Instead of keeping an array of booleans, we now hang onto an array of
isl_aux_usage enums.  This means that the thing we are passing from
brw_draw.c to surface state setup is the thing that surface state setup
actually needs instead of an input to compute what it needs.

Cc: mesa-stable@lists.freedesktop.org
Reviewed-by: Topi Pohjolainen <topi.pohjolainen@intel.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
(cherry picked from commit e52a9f18)