tools/power turbostat: show GFXMHz

BugLink: http://bugs.launchpad.net/bugs/1591802



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>
(cherry picked from commit 27d47356)
Signed-off-by: Tim Gardner <tim.gardner@canonical.com>
Acked-by: Brad Figg <brad.figg@canonical.com>
Acked-by: Kamal Mostafa <kamal@canonical.com>