intel/fs, i965, iris: drop unused binding table entries

Keep track and remove unused binding table entries. In few cases this tracking will also prevent the driver to upload an unused SURFACE_STATE -- this is nowadays handled per surface group (not for all of them) via other means.

The key change here is to pass around not a surface index, but an index and a group, e.g. Texture #3 (closed) instead of final index 4. It currently works by adding an extra pass before the conversion from NIR to backend instructions. Would be best if this was handled later, but that would require a few more changes in the code generation.

The first motivation for the changes was kwg/mesa#15 (closed). I will check, but it's likely that this MR will do the trick there.

I initially thought the improvement mentioned above was a requirement for this MR, but since the changes as is seem to be able to help in the Iris case (and also help existing i965), I'm leaning to first land (some evolution of) this MR and then work on the improvement in a separate MR.

Thanks to @kwg, @jekstrand and @rantogno for helping out with my seemingly random questions about Gen.