i965: Replace bool aux disable with enum

i965: Replace bool aux disable with enum

As CCS buffers are passed to KMS, it becomes useful to be able to
determine exactly what type of aux buffers are disabled. This was
previously not entirely needed (though the code was a little more
confusing), however it becomes very desirable after a recent patch from
Chad:

commit 1c8be049bea786c2c054a770025976beba5b8636
Author: Chad Versace <chadversary@chromium.org>
Date:   Fri Dec 9 16:18:11 2016 -0800

    i965/mt: Disable aux surfaces after making miptree shareable

The next patch will handle CCS and get rid of no_ccs.

Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
Reviewed-by: Topi Pohjolainen <topi.pohjolainen@intel.com>