powerpc/pmac/smp: Fixup smp_core99_cpu_disable() and use it on 64-bit

Use the generic code, just add the MPIC priority setting,

I don't see any use in mucking around with the decrementer,
as 32-bit will have EE off all along, and 64-bit will be able
to deal with it.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>