intel/compiler: Refactor TCS invocation ID setup into a helper

When we add 8_PATCH mode, this will get a bit more complex, so we may
as well start by putting it in a helper function.

Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>

src/intel/compiler/brw_fs.h

@@ -194,6 +194,8 @@ public:
    bool opt_cmod_propagation();
    bool opt_zero_samples();
 
+   void set_tcs_invocation_id();
+
    void emit_nir_code();
    void nir_setup_outputs();
    void nir_setup_uniforms();