ppc/xive2: Avoid needless interrupt re-check on CPPR set

ppc/xive2: Avoid needless interrupt re-check on CPPR set

When CPPR priority is decreased, pending interrupts do not need to be
re-checked if one is already presented because by definition that will
be the highest priority.

This prevents a presented group interrupt from being lost.

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/20250512031100.439842-39-npiggin@gmail.com
Signed-off-by: Cédric Le Goater <clg@redhat.com>