intel/compiler: use 0 as sampler in emit_mcs_fetch

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>