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