powerpc: Partial revert of "Context switch more PMU related SPRs"

In commit 59affcd3 I added context switching of more PMU SPRs, because
they are potentially exposed to userspace on Power8. However despite me
being a smart arse in the commit message it's actually not correct. In
particular it interacts badly with a global perf record.

We will have to do something more complicated, but that will have to
wait for 3.11.

Signed-off-by: Michael Ellerman <michael@ellerman.id.au>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>