drm/xe: Rename xe_gt_idle_sysfs to xe_gt_idle

Prep this file to contain C6 toggling as well instead
of just sysfs related stuff.

Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Vinay Belgaumkar <vinay.belgaumkar@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>