radeonsi/gfx10: don't initialize VGT_INSTANCE_STEP_RATE_0

Only gfx9 and older use it to get InstanceID in VGPR1.

src/gallium/drivers/radeonsi/si_state.c

@@ -5436,7 +5436,8 @@ static void si_init_config(struct si_context *sctx)
 		si_pm4_set_reg(pm4, R_028B98_VGT_STRMOUT_BUFFER_CONFIG, 0x0);
 	}
 
-	si_pm4_set_reg(pm4, R_028AA0_VGT_INSTANCE_STEP_RATE_0, 1);
+	if (sscreen->info.chip_class <= GFX9)
+		si_pm4_set_reg(pm4, R_028AA0_VGT_INSTANCE_STEP_RATE_0, 1);
 	if (!has_clear_state)
 		si_pm4_set_reg(pm4, R_028AB8_VGT_VTX_CNT_EN, 0x0);
 	if (sctx->chip_class < GFX7)