perf, intel: Try alternative OFFCORE encodings

Since the OFFCORE registers are fully symmetric, try the other one
when the specified one is already in use.

Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/r/1306141897.18455.8.camel@twins


Signed-off-by: Ingo Molnar <mingo@elte.hu>