Drop the default for performance_cx_lowest (ie, what to use when AC is connected) to ACPI C2.

ACPI C3 ends up doing a lot more work before entering sleep, some of which
requires grabbing a global ACPI hardware serialising mutex.

Because of this, the more CPU cores you have, the more that lock contends
under load, reaching close to the #1 lock contention (after VM, which is being
worked on.)

Tested:

• Sandy bridge Xeon, 2 socket * 8 core
• Ivy bridge Xeon v2, 2 socket * 8 core
• Westmere-EX, 4 socket * 10 core
• Ivybridge desktop
• Sandybridge mobile
• Ivybridge mobile

MFC after: 2 weeks