ACPI: ibm_acpi: Remove experimental status for brightness and volume.

The brightness and volume features from ibm-acpi are stable.
The experimental flag is no longer needed.

Signed-off-by: Stefan Schmidt <stefan@datenfreihafen.org>
Acked-by: Borislav Deianov <borislav@users.sourceforge.net>
Signed-off-by: Len Brown <len.brown@intel.com>