i965: Use ISL for emitting depth/stencil/hiz state on gen6+

i965: Use ISL for emitting depth/stencil/hiz state on gen6+

We leave gen4-5 alone because the ISL code hasn't really been well-
tested on gen4-5 or with combined depth-stencil because we don't use
BLORP for depth operations on gen4-5.  Also, the gen4-5 code has to deal
with intratile offsets for LOD hacks and ISL doesn't handle those yet.
We could make ISL handle gen4-5 capable or we could just not bother.

Among other things, this should make future platform enabling easier
because it means we don't have to update multiple (or hand-rolled!)
depth stencil emit paths.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>