tools/power turbostat: show GFX%rc6

The column "GFX%c6" show the percentage of time the GPU
is in the "render C6" state, rc6.  Deep package C-states on several
systems depend on the GPU being in RC6.

This information comes from the counter
/sys/class/drm/card0/power/rc6_residency_ms,
as read before and after the measurement interval.

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