tools/power/turbostat: Cache graphics sysfs path

Graphics drivers (i915/Xe) have different sysfs knobs on different
platforms, and it is possible that different sysfs knobs fit into the
same turbostat columns.

Instead of specifying different sysfs knobs every time, detect them
once and cache the path for future use.

No functional change.

Signed-off-by: Zhang Rui <rui.zhang@intel.com>
Signed-off-by: Len Brown <len.brown@intel.com>