x86: warn of incorrect cpu_khz on AMD systems

Impact: add debug check

If none of the perfctrs are free when calculating cpu_khz we default to
using ctr 3 (ie, we just choose 3).  This may lead to an incorrect tsc
freq value which can cause the system to be unstable.

To aid in future debugging, WARN the user of a potential problem.

Signed-off-by: Prarit Bhargava <prarit@redhat.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>