powerpc/mm: Used free register to save a few cycles in SW TLB miss handling

Now that r0 is free we can keep the value of I/DMISS in r3 and not reload
it before doing the tlbli/d.  This saves us a few cycles in the fast path
case.

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