intel/compiler: Create and use struct for GS thread payload

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Marcin Ślusarz <marcin.slusarz@intel.com>
Acked-by: Ian Romanick <ian.d.romanick@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/18176>

src/intel/compiler/brw_fs_visitor.cpp

@@ -778,7 +778,7 @@ fs_visitor::emit_urb_writes(const fs_reg &gs_vertex_count)
       urb_handle = tes_payload().urb_output;
       break;
    case MESA_SHADER_GEOMETRY:
-      urb_handle = fs_reg(retype(brw_vec8_grf(1, 0), BRW_REGISTER_TYPE_UD));
+      urb_handle = gs_payload().urb_handles;
       break;
    default:
       unreachable("invalid stage");