intel/fs: Drop the gl_program from fs_visitor

It's not used by anything anymore now that so much lowering has been
moved into NIR.  Sadly, we still need on in brw_compile_gs() for
geometry shaders on Sandy Bridge.  Short of a lot of pointless work,
that one's probably not going away.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>

src/intel/compiler/test_fs_saturate_propagation.cpp

@@ -47,8 +47,7 @@ public:
                                    struct brw_wm_prog_data *prog_data,
                                    nir_shader *shader)
       : fs_visitor(compiler, NULL, NULL, NULL,
-                   &prog_data->base, (struct gl_program *) NULL,
-                   shader, 16, -1) {}
+                   &prog_data->base, shader, 16, -1) {}
 };