ppc/xive2: fix context push calculation of IPB priority

ppc/xive2: fix context push calculation of IPB priority

Pushing a context and loading IPB from NVP is defined to merge ('or')
that IPB into the TIMA IPB register. PIPR should therefore be calculated
based on the final IPB value, not just the NVP value.

Fixes: 9d2b6058c5b ("ppc/xive2: Add grouping level to notification")
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Reviewed-by: Glenn Miles <milesg@linux.ibm.com>
Reviewed-by: Caleb Schlossin <calebs@linux.ibm.com>
Tested-by: Gautam Menghani <gautam@linux.ibm.com>
Link: https://lore.kernel.org/qemu-devel/20250512031100.439842-6-npiggin@gmail.com
Signed-off-by: Cédric Le Goater <clg@redhat.com>