UefiCpuPkg/MpLib.c: Set AP state after X2APIC mode enabled

UefiCpuPkg/MpLib.c: Set AP state after X2APIC mode enabled

After X2APIC mode is enabled, APs need to be set tp IDLE state, otherwise APs
cannot be waken up by MP PPI services.

https://bugzilla.tianocore.org/show_bug.cgi?id=505

Cc: Feng Tian <feng.tian@intel.com>
Cc: Michael Kinney <michael.d.kinney@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Jeff Fan <jeff.fan@intel.com>
Reviewed-by: Feng Tian <feng.tian@intel.com>