powerpc/64/interrupt: reduce expensive debug tests

Move the assertions requiring restart table searches under
CONFIG_PPC_IRQ_SOFT_MASK_DEBUG.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20210922145452.352571-6-npiggin@gmail.com