powerpc: Fix mpic_resume on early G5 macs

mpic_resume() on G5 macs blindly dereferences mpic->fixups, but
it may legitimately be NULL (as on PowerMac7,2).  Add an explicit
check.

This fixes suspend-to-disk with one processor (maxcpus=1) for me.

Signed-off-by: Alastair Bridgewater <alastair.bridgewater@gmail.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>