powerpc/perf: Use existing out label in power_pmu_enable()

In power_pmu_enable() we can use the existing out label to reduce the
number of return paths.

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>