mesa: merge bind_xfb_buffers_{base|range}
mesa: merge bind_xfb_buffers_{base|range}

Reduced code duplication should make the code more maintainable.

Reviewed-by: Ian Romanick <2b237cafb16dc45038e85df6c85e74e6d899eba9@intel.com>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler