x86/apic: Make disable x2apic work really

If x2apic_preenabled is not enabled, then disable_x2apic() is not
called from various places which results in x2apic_disabled not being
set. So other code pathes can happily reenable the x2apic.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Jiang Liu <jiang.liu@linux.intel.com>
Cc: Joerg Roedel <joro@8bytes.org>
Cc: Tony Luck <tony.luck@intel.com>
Cc: Borislav Petkov <bp@alien8.de>
Link: http://lkml.kernel.org/r/20150115211702.621431109@linutronix.de
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>