[POWERPC] Push down or eliminate smp_processor_id calls in xics code

The per-processor interrupt request register and current processor
priority register are only accessed on the current cpu.  In fact the
hypervisor doesn't even let us choose which cpu's registers to access.

The only function to use cpu twice is xics_migrate_irqs_away, not a fast
path.  But we can cache the result of get_hard_processor_id() instead of
calling get_hard_smp_processor_id(cpu) in a loop across the call to rtas.

Years ago the irq code passed smp_processor_id into get_irq, I thought
we might initialize the CPPR third party at boot as an extra measure of
saftey, and it made the code symmetric with the qirr (queued interrupt
for software generated interrupts), but now it is just extra and
sometimes unneeded work to pass it down.

Signed-off-by: Milton Miller <miltonm@bga.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>