tools/power/turbostat: Support alternative graphics sysfs knobs

tools/power/turbostat: Support alternative graphics sysfs knobs

/sys/class/graphics/fb0/device/drm/card0/ and /sys/class/drm/card0/
point to the same device node.
But in some cases, one exists and the other one does not.

Prefer to use /sys/class/drm/card0/, and fall back to
/sys/class/graphics/fb0/device/drm/card0/.

This recovers the "GFXMHz" and "GFXAMHz" columns on some platforms like
a SPR server.

Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Zhang Rui <rui.zhang@intel.com>
Reviewed-by: Len Brown <len.brown@intel.com>