tools/power turbostat: show GFXMHz

tools/power turbostat: show GFXMHz

Orabug: 24811361

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 27d47356b6dfa92042a17a0b474f08910d4c8e8f)
Signed-off-by: Brian Maly <brian.maly@oracle.com>
Signed-off-by: Dhaval Giani <dhaval.giani@oracle.com>