powerpc/perf: Pass the struct perf_events down to compute_mmcr()

To support per-event exclude settings on Power8 we need access to the
struct perf_events in compute_mmcr().

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