ff_fragment_shader: Use binding to set the sampler unit

ff_fragment_shader: Use binding to set the sampler unit

This is the way layout(binding=xxx) works from GLSL.  The old method
just happened to work (and significantly predated support for
layout(binding=xxx)), but future changes will break this.

v2: Remove some stale comments.  Suggested by Matt and Chris Forbes.

Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Matt Turner <mattst88@gmail.com>
Cc: "10.6 11.0" <mesa-stable@lists.freedesktop.org>