drm/i915/vlv: use a lower RC6 timeout on VLV

We use timeout mode, and we need to lower the timeout to get good RC6
residency when loads are running.  This gets me from 0% residency during
glxgears to 77%, which is a pretty good improvement.  This value also
matches the current BWG recommentations.
Tested-by: "Meng, Mengmeng" <mengmeng.meng@intel.com>
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Reviewed-by: Deepak S <deepak.s@inel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/intel_pm.c

@@ -4110,7 +4110,7 @@ static void valleyview_enable_rps(struct drm_device *dev)
 	for_each_ring(ring, dev_priv, i)
 		I915_WRITE(RING_MAX_IDLE(ring->mmio_base), 10);
 
-	I915_WRITE(GEN6_RC6_THRESHOLD, 0xc350);
+	I915_WRITE(GEN6_RC6_THRESHOLD, 0x557);
 
 	/* allows RC6 residency counter to work */
 	I915_WRITE(VLV_COUNTER_CONTROL,