powerpc/book3e: Don't re-trigger decrementer on lazy irq restore

The decrementer on BookE acts as a level interrupt and doesn't
need to be re-triggered when going negative. It doesn't go
negative anyways (unless programmed to auto-reload with a
negative value) as it stops when reaching 0.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>