tools/power turbostat: Cache graphics sysfs file descriptors during probe

Snapshots of the graphics sysfs knobs are taken based on file
descriptors. To optimize this process, open the files and cache the file
descriptors during the graphics probe phase. As a result, the previously
cached pathnames become redundant and are removed.

This change aims to streamline the code without altering its functionality.

No functional change intended.

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