intel/gen6: Some framebuffers having separate depthstencil should be unsupported

intel/gen6: Some framebuffers having separate depthstencil should be unsupported

When the framebuffer has separate depth and stencil buffers, and HiZ is
not enabled on the depth buffer, mark the framebuffer as unsupported. This
happens when trying to create a framebuffer with Z16/S8 because we haven't
enabled HiZ on Z16 yet.

Fixes gles2conform test stencil8.

Note: This is a candiate for the 8.0 branch.
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=44948
Reviewed-and-tested-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed--by: Eric Anholt <eric@anholt.net>
Signed-off-by: Chad Versace <chad.versace@linux.intel.com>