tools/power turbostat: Consolidate graphics sysfs access

Currently, there is an inconsistency in how graphics sysfs knobs are
accessed: graphics residency sysfs knobs are opened and closed for each
read, while graphics frequency sysfs knobs are opened once and remain
open until turbostat exits. This inconsistency is confusing and adds
unnecessary code complexity.

Consolidate the access method by opening the sysfs files once and
reusing the file pointers for subsequent accesses. This approach
simplifies the code and ensures a consistent method for accessing
graphics sysfs knobs.

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