i965/gen9: Add HiZ auxiliary buffer support

i965/gen9: Add HiZ auxiliary buffer support

The original functionality this patch introduces was authored by a patch from
Ken (the commit subject was the same). Since I ended up changing so many patches
in the code before this one, I had some non-trivial decisions to make, and I
didn't feel it was appropriate to keeps Ken's name as author (mostly because he
might not like what I've done). Ken's original patch was like 2 LOC :-)

In either case, some credit needs to go to Ken, and to Jordan for a few small
other changes in that original patch.

v2: Back to a smaller diff now that ISL handles most of the actual
    programming (Lionel)

Signed-off-by: Ben Widawsky <benjamin.widawsky@intel.com> (v1)
Signed-off-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com> (v2)
Reviewed-by: Topi Pohjolainen <topi.pohjolainen@intel.com>