x86: refactor ->enable_apic_mode() subarch methods

Only ES7000 has a real ->enable_apic_mode() method, the other
subarchitectures define it but keep it empty.

So mark the vector as NULL, extend the generic code to handle
NULL -setup_portio_remap() entries and remove all the empty
handlers.

Signed-off-by: Ingo Molnar <mingo@elte.hu>
diff --git a/arch/x86/mach-generic/bigsmp.c b/arch/x86/mach-generic/bigsmp.c
index 82743d1..e151b47 100644
--- a/arch/x86/mach-generic/bigsmp.c
+++ b/arch/x86/mach-generic/bigsmp.c
@@ -86,7 +86,7 @@
 	.apicid_to_cpu_present		= bigsmp_apicid_to_cpu_present,
 	.setup_portio_remap		= NULL,
 	.check_phys_apicid_present	= bigsmp_check_phys_apicid_present,
-	.enable_apic_mode		= enable_apic_mode,
+	.enable_apic_mode		= NULL,
 	.phys_pkg_id			= phys_pkg_id,
 	.mps_oem_check			= mps_oem_check,