tools/power turbostat: Restore GFX sysfs fflush() call

Do fflush() to discard the buffered data, before each read of the
graphics sysfs knobs.

Fixes: ba99a4fc ("tools/power turbostat: Remove unnecessary fflush() call")
Signed-off-by: Zhang Rui <rui.zhang@intel.com>
Signed-off-by: Len Brown <len.brown@intel.com>