ppc/xive2: Consolidate presentation processing in context push

ppc/xive2: Consolidate presentation processing in context push

OS-push operation must re-present pending interrupts. Use the
newly created xive2_tctx_process_pending() function instead of
duplicating the 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/20250512031100.439842-38-npiggin@gmail.com
Signed-off-by: Cédric Le Goater <clg@redhat.com>