powerpc: Re-order setup_panic()

Do it right after probe_machine() since it's about testing ppc_md,
and put the test in the common code.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/kernel/setup-common.c

@@ -688,6 +688,8 @@ static struct notifier_block ppc_panic_block = {
 
 void __init setup_panic(void)
 {
+	if (!ppc_md.panic)
+		return;
 	atomic_notifier_chain_register(&panic_notifier_list, &ppc_panic_block);
 }