linker: Various trivial clean-ups in set_sampler_binding

linker: Various trivial clean-ups in set_sampler_binding

- Remove the spurious block left from the previous commit and re-indent.

- Constify elements.

- Make the spec reference in the code look like other spec references in
  the compiler.

Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=76323
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Cc: "10.1" <mesa-stable@lists.freedesktop.org>
Cc: github@socker.lepus.uberspace.de