ppc/xive: Split xive recompute from IPB function

ppc/xive: Split xive recompute from IPB function

Further split xive_tctx_pipr_update() by splitting out a new function
that is used to re-compute the PIPR from IPB. This is generally only
used with XIVE1, because group interrputs require more logic.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Reviewed-by: Glenn Miles <milesg@linux.ibm.com>
Reviewed-by: Michael Kowal <kowal@linux.ibm.com>
Tested-by: Gautam Menghani <gautam@linux.ibm.com>
Link: https://lore.kernel.org/qemu-devel/20250512031100.439842-33-npiggin@gmail.com
Signed-off-by: Cédric Le Goater <clg@redhat.com>