x86, x2apic: Don't map lapic addr for preenabled x2apic systems

If x2apic is preenabled and used by the kernel, we don't need to map
the lapic address. That mapping will never be used.

So just skip that in register_lapic_address()

Signed-off-by: Yinghai Lu <yinghai@kernel.org>
Cc: Suresh Siddha <suresh.b.siddha@intel.com>
Cc: "Eric W. Biederman" <ebiederm@xmission.com>
LKML-Reference: <4CFDF69C.9070501@kernel.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>