drm/i915: Set softmin frequency on idle->busy transition

If the GPU load is low enough, it's possible that we'll be stuck at idle
frequency rather than transition into softmin frequency requested by
userspace.

v2: Use intel_set_rps, drop vlv_set_idle
v3: Back to vlv_set_idle, clamp to valid range
v4: Place intel_set_rps at the end

References: https://bugs.freedesktop.org/show_bug.cgi?id=89728
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Imre Deak <imre.deak@intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Michał Winiarski <michal.winiarski@intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: http://patchwork.freedesktop.org/patch/msgid/1466416707-12075-1-git-send-email-michal.winiarski@intel.com

drivers/gpu/drm/i915/intel_pm.c

@@ -4844,6 +4844,12 @@ void gen6_rps_busy(struct drm_i915_private *dev_priv)
 			gen6_rps_reset_ei(dev_priv);
 		I915_WRITE(GEN6_PMINTRMSK,
 			   gen6_rps_pm_mask(dev_priv, dev_priv->rps.cur_freq));
+
+		/* Ensure we start at the user's desired frequency */
+		intel_set_rps(dev_priv,
+			      clamp(dev_priv->rps.cur_freq,
+				    dev_priv->rps.min_freq_softlimit,
+				    dev_priv->rps.max_freq_softlimit));
 	}
 	mutex_unlock(&dev_priv->rps.hw_lock);
 }