tools/power turbostat: show GFXMHz

Under the column "GFXMHz", show a snapshot of this attribute:
/sys/class/graphics/fb0/device/drm/card0/gt_cur_freq_mhz

This is an instantaneous snapshot of what sysfs presents
at the end of the measurement interval.  turbostat does
not average or otherwise perform any math on this value.

Signed-off-by: Len Brown <len.brown@intel.com>