perf, x86: Calculate perfctr msr addresses in helper functions

This patch adds helper functions to calculate perfctr msr addresses.
We need this to later add support for AMD family 15h cpus. For this we
have to change the algorithms to generate the perfctr's msr addresses.

Signed-off-by: Robert Richter <robert.richter@amd.com>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
LKML-Reference: <1296664860-10886-3-git-send-email-robert.richter@amd.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>