intel/compiler: Use FS thread payload only for FS

Move the setup into the FS thread payload constructor.  Consolidate
payload setup for that in brw_fs_thread_payload.cpp file.

Reviewed-by: Francisco Jerez <currojerez@riseup.net>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/18176>

src/intel/compiler/brw_vec4.cpp

@@ -2642,7 +2642,7 @@ brw_compile_vs(const struct brw_compiler *compiler,
          return NULL;
       }
 
-      prog_data->base.base.dispatch_grf_start_reg = v.payload.num_regs;
+      prog_data->base.base.dispatch_grf_start_reg = v.payload().num_regs;
 
       fs_generator g(compiler, params->log_data, mem_ctx,
                      &prog_data->base.base, v.runtime_check_aads_emit,