powerpc: Fix power4/970 idle code regression with lockdep

in commit 7230c5644188cd9e3fb380cc97dde00c464a3ba7
"powerpc: Rework lazy-interrupt handling"

I introduced a regression, accidentally calling irq tracing twice
and not properly restoring a clobbered register (r7) later used
for writing to the MSR.

This caused lockups when booting on a G5 with lockdep enabled.

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