x86/apic: Mark the apic_intr_mode extern for sanity check cleanup

Calling native_smp_prepare_cpus() to prepare for SMP bootup, does some
sanity checking, enables APIC mode and disables SMP feature.

Now, APIC mode setup has been unified to apic_intr_mode_init(), some sanity
checks are redundant and need to be cleanup.

Mark the apic_intr_mode extern to refine the switch and remove the
redundant sanity check.

Signed-off-by: Dou Liyang <douly.fnst@cn.fujitsu.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: yinghai@kernel.org
Cc: bhe@redhat.com
Link: https://lkml.kernel.org/r/1505293975-26005-7-git-send-email-douly.fnst@cn.fujitsu.com