Now that everything has apic::max_apic_id set and the eventual update for
the x2APIC case is in place, switch the apic_id_valid() helper to use
apic::max_apic_id and remove the apic::apic_id_valid() callback.

[ dhansen: Fix subject typo ]
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)