fix bogus assumption if ddx has set up surface reg for z buffer

fix bogus assumption if ddx has set up surface reg for z buffer

this is wrong since even if ddx has not set up a surface reg to cover the z
buffer we should pretend it has on those rv100 chips since they presumably do
not do z buffer tiling if not using hyperz, so we can use linear addressing
just the same. Doesn't seem to fix #13080, but it's wrong anyway and the bug
almost certainly broke newer non-tcl chips.