ACPI: Defer enabling of level GPE until all pending notifies done

Level GPE should not be enabled until all work caused by it is done,
e.g. all Notify() methods are completed.
This can be accomplished by appending enable_gpe function to the end
of notify queue.

Signed-off-by: Alexey Starikovskiy <astarikovskiy@suse.de>
Acked-by: Shaohua Li <shaohua.li@intel.com>
Signed-off-by: Len Brown <len.brown@intel.com>