intel: Use separate stencil whenever possible

intel: Use separate stencil whenever possible

For depthstencil renderbuffers, we were using separate stencil only if the
hardware required it. Since the performance gains from HiZ is so high, we
should always use separate stencil if the hardware supports it.

Reviewed-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Chad Versace <chad.versace@linux.intel.com>