i965: Don't depth test the fake depthbuffer when one isn't present.

i965: Don't depth test the fake depthbuffer when one isn't present.

For the non-separate-stencil-only case, we've been using a NULL
surface for depth, so we didn't have to care.  However, to support
separate stencil with no depthbuffer, we have to make the depth
surface non-NULL or the stencil test always fails thanks to separate
stencil inheriting the surface type of depth.

Fixes hiz-depth-stencil-test-d0-s8.

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