intel/compiler: fix shading rate in mesh shaders

brw_nir_lower_shading_rate_output is already called in brw_postprocess_nir and calling it the second time actually breaks shading rate.

Initially, when I added this call here in 9acb30c8, I was testing it on an internal tree, which didn't have brw_nir_lower_shading_rate_output call in brw_postprocess_nir.