ACPICA: Events: Back port "ACPICA: Save current masks of enabled GPEs after enable register writes"

This is a back port result of the Linux commit:
  Commit c50f13c6
  Subject: ACPICA: Save current masks of enabled GPEs after enable register writes

Besides of the indent divergences, only a missing prototype added due to
the ACPICA internal coding style.

Signed-off-by: Lv Zheng <lv.zheng@intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>