drm/i915: Keep min freq above floor on HSW/BDW

Set the min_freq_softlimit to max(RPe, 450MHz).

Setting a floor can ensure a minimum experience
level.  The 450MHz value came from a power and
performance study of various types of workloads
(3D, Media, GPGPU, idle, etc).

v2: rebased
Signed-off-by: Tom O'Rourke <Tom.O'Rourke@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/intel_pm.c

@@ -4643,7 +4643,8 @@ static void gen6_init_rps_frequencies(struct drm_device *dev)
 	if (dev_priv->rps.min_freq_softlimit == 0) {
 		if (IS_HASWELL(dev) || IS_BROADWELL(dev))
 			dev_priv->rps.min_freq_softlimit =
-				dev_priv->rps.efficient_freq;
+				/* max(RPe, 450 MHz) */
+				max(dev_priv->rps.efficient_freq, (u8) 9);
 		else
 			dev_priv->rps.min_freq_softlimit =
 				dev_priv->rps.min_freq;