ppc/xive2: split tctx presentation processing from set CPPR

ppc/xive2: split tctx presentation processing from set CPPR

The second part of the set CPPR operation is to process (or re-present)
any pending interrupts after CPPR is adjusted.

Split this presentation processing out into a standalone function that
can be used in other places.

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/20250512031100.439842-37-npiggin@gmail.com
Signed-off-by: Cédric Le Goater <clg@redhat.com>