radeonsi/gfx9: don't read back non-existent register SRBM_STATUS2

It looks like there is no way to monitor SDMA busyness on GFX9.

Reviewed-by: Nicolai Hähnle <nicolai.haehnle@amd.com>

src/gallium/drivers/radeon/r600_gpu_load.c

@@ -105,7 +105,7 @@ static void r600_update_mmio_counters(struct r600_common_screen *rscreen,
 	UPDATE_COUNTER(gui, GUI_ACTIVE);
 	gui_busy = GUI_ACTIVE(value);
 
-	if (rscreen->chip_class >= CIK) {
+	if (rscreen->chip_class == CIK || rscreen->chip_class == VI) {
 		/* SRBM_STATUS2 */
 		rscreen->ws->read_registers(rscreen->ws, SRBM_STATUS2, 1, &value);