[POWERPC] PMU: Don't lock_kernel()

I see nothing that this lock_kernel() actually protects against,
so remove it.

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Acked-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>