ppc/xive: Add xive_tctx_pipr_set() helper function

ppc/xive: Add xive_tctx_pipr_set() helper function

Have xive_tctx_notify() also set the new PIPR value and rename it to
xive_tctx_pipr_set(). This can replace the last xive_tctx_pipr_update()
caller because it does not need to update IPB (it already sets it).

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/20250512031100.439842-36-npiggin@gmail.com
Signed-off-by: Cédric Le Goater <clg@redhat.com>