drm/i915: Fix gpu frequency change tracing

With gen < 9 we have had always 50Mhz units as our hw
ratio. With gen >= 9 the hw ratio changed to 16.667Mhz (50/3).
The result was that our gpu frequency tracing started to output
values 3 times larger than expected due to hardcoded scaling
value. Fix this by using  Use intel_gpu_freq() when generating Mhz
value from ratio for 'intel_gpu_freq_change' trace event.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=92591
Cc: stable@vger.kernel.org # v4.3+
Reported-by: Eero Tamminen <eero.t.tamminen@intel.com>
Signed-off-by: Mika Kuoppala <mika.kuoppala@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1447776866-29384-1-git-send-email-mika.kuoppala@intel.com

drivers/gpu/drm/i915/intel_pm.c

@@ -4449,7 +4449,7 @@ static void gen6_set_rps(struct drm_device *dev, u8 val)
 	POSTING_READ(GEN6_RPNSWREQ);
 
 	dev_priv->rps.cur_freq = val;
-	trace_intel_gpu_freq_change(val * 50);
+	trace_intel_gpu_freq_change(intel_gpu_freq(dev_priv, val));
 }
 
 static void valleyview_set_rps(struct drm_device *dev, u8 val)