powerpc/perf: Clear all MMCR settings before calling compute_mmcr()

Because we reuse cpuhw->mmcr on each call to compute_mmcr() there's a
risk that we could forget to set one of the values and use whatever
value was in there previously.

Currently all the implementations are careful to set all the values, but
it's safer to clear them all before we call compute_mmcr().

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