intel/blorp: Emit a NULL render target for depth/stencil-only operations

intel/blorp: Emit a NULL render target for depth/stencil-only operations

This never mattered before because the only time we used blorp
depth/stencil only was to do HiZ operations on gen6-7.  It may have worked
in that case (and maybe it didn't) but slow depth clears actually do depth
rendering so they need a valid render target.

Signed-off-by: Jason Ekstrand <jason@jlekstrand.net>
Reviewed-by: Topi Pohjolainen <topi.pohjolainen@intel.com>