intel/compiler: use 0 as sampler in emit_mcs_fetch

The sampler will be ignored since the underlying 'ld_mcs' operation
won't use it, so just fill the field with 0 instead of the texture to
make it clearer that's the case.

This will also avoid is_high_sampler() to kick in unnecessarily, in
case we are using the operation for a texture with index >= 16.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>

src/intel/compiler/brw_vec4_visitor.cpp

@@ -863,7 +863,7 @@ vec4_visitor::emit_mcs_fetch(const glsl_type *coordinate_type,
                                     dst_reg(this, glsl_type::uvec4_type));
    inst->base_mrf = 2;
    inst->src[1] = surface;
-   inst->src[2] = surface;
+   inst->src[2] = brw_imm_ud(0); /* sampler */
 
    int param_base;