intel: Only prefer separate stencil when we can do HiZ.

intel: Only prefer separate stencil when we can do HiZ.

This required is_hiz_depth_format to start returning true on S8_Z24 as
well, since that's the format we have here.  The two previous callers
are only calling it on non-depthstencil formats.

This avoids us needing to have HiZ working on a new Z format
immediately upon exposing the format (particularly painful for
Z32_FLOAT_X24S8, which means all the fake packed depth/stencil paths).

Reviewed-by: Chad Versace <chad.versace@linux.intel.com>