i965/gen6: Force ALL_SLICES_AT_EACH_LOD for separate stencil/hiz

i965/gen6: Force ALL_SLICES_AT_EACH_LOD for separate stencil/hiz

For gen6 we will use the ALL_SLICES_AT_EACH_LOD miptree layout for
separate stencil/hiz. This is needed because gen6 hiz and separate
stencil only support a single miplevel. When accessing the other LODs,
we will program a tile aligned offset for the bo.

PRM Volume 1, Part 1, 7.18.3.7.2 For separate stencil buffer [DevILK]
to [DevSNB]:
 "The separate stencil buffer does not support mip mapping, thus the
  storage for LODs other than LOD 0 is not needed."

We still allocate storage for the other stencil mip-levels within a
single texture, but each mip-level will use non-mip-array spacing.

PRM Volume 2, Part 1, 7.5.3 Hierarchical Depth Buffer
 "[DevSNB]: The hierarchical depth buffer does not support the LOD
  field, it is assumed by hardware to be zero. A separate
  hierarachical depth buffer is required for each LOD used, and the
  corresponding buffer’s state delivered to hardware each time a new
  depth buffer state with modified LOD is delivered."

We allocate storage for the other hiz mip-levels within a single
texture, but each mip-level will use non-mip-array spacing.

Signed-off-by: Jordan Justen <jordan.l.justen@intel.com>
Reviewed-by: Topi Pohjolainen <topi.pohjolainen@intel.com>
Acked-by: Kenneth Graunke <kenneth@whitecape.org>