i965/miptree: Rework aux enabling

i965/miptree: Rework aux enabling

This commit replaces the complex and confusing set of disable flags with
two fairly straightforward fields which describe the intended auxiliary
surface usage and whether or not the miptree supports fast clears.
Right now, supports_fast_clear can be entirely derived from aux_usage
but that will not always be the case.

This commit makes functional changes.  One of these changes is that it
re-enables multisampled fast-clears which were accidentally disabled in
cec30a666930ddb8476a9452a89364a24979ff62 around a year ago.  Fixing this
improves the SynMark v7 DeferredAA test by around ~3% on some gen9
hardware.  This commit also gets us closer to enabling CCS_E for
window-system buffers which are Y-tiled.

Reviewed-by: Chad Versace <chadversary@chromium.org>
Reviewed-by: Topi Pohjolainen <topi.pohjolainen@intel.com>