powerpc/perf: Freeze PMC5/6 if we're not using them

On Power8 we can freeze PMC5 and 6 if we're not using them. Normally they
run all the time.

As noticed by Anshuman, we should unfreeze them when we disable the PMU
as there are legacy tools which expect them to run all the time.

Signed-off-by: Michael Ellerman <michael@ellerman.id.au>
CC: <stable@vger.kernel.org> [v3.10]
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>