x86/x2apic: Add proper state tracking

Having 3 different variables to track the state is just silly and
error prone. Add a proper state tracking variable which covers the
three possible states: ON/OFF/DISABLED.

We cannot use x2apic_mode for this as this would require to change all
users of x2apic_mode with explicit comparisons for a state value
instead of treating it as boolean.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Borislav Petkov <bp@alien8.de>
Cc: Jiang Liu <jiang.liu@linux.intel.com>
Cc: Joerg Roedel <joro@8bytes.org>
Cc: Tony Luck <tony.luck@intel.com>
Link: http://lkml.kernel.org/r/20150115211702.955392443@linutronix.de


Signed-off-by: Thomas Gleixner <tglx@linutronix.de>