intel/fs: Use image_deref intrinsics instead of image_var

intel/fs: Use image_deref intrinsics instead of image_var

Since we had to rewrite the deref walking loop anyway, I took the
opportunity to make it a bit clearer and more efficient.  In particular,
in the AoA case, we will now emit one minmax instead of one per array
level.

Acked-by: Rob Clark <robdclark@gmail.com>
Acked-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>
Acked-by: Dave Airlie <airlied@redhat.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>