x86/apic: Split register_apic_address()

Split the fixmap setup out of register_lapic_address() and reuse it when
the X2APIC is disabled during setup.

This avoids registering the APIC ID (setting 'mp_lapic_addr') twice.

[ dhansen: changelog wording tweak ]

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Tested-by: Michael Kelley <mikelley@microsoft.com>
Tested-by: Sohil Mehta <sohil.mehta@intel.com>
Tested-by: Juergen Gross <jgross@suse.com> # Xen PV (dom0 and unpriv. guest)