x86/pmu: Add helper to get fixed counter MSR index

Add a helper to get the index of a fixed counter instead of manually
calculating the index, a future patch will add more users of the fixed
counter MSRs.

No functional change intended.

Signed-off-by: Like Xu <likexu@tencent.com>
[sean: move to separate patch, write changelog]
Signed-off-by: Sean Christopherson <seanjc@google.com>
Link: https://lore.kernel.org/r/20221102225110.3023543-20-seanjc@google.com