x86/cpufreq: fix turbo mode detection

x86/cpufreq: fix turbo mode detection

{acpi,powernow}_cpufreq_cpu_init() generally don't run on the CPU the
policy they deal with is related to, hence using cpuid() directly
works only as long as all CPUs in the system are identical (which
admittedly is commonly the case).

Further add a per-policy flag indicating the availability of
APERF/MPERF MSRs, so that globally setting the .getavg accessor won't
be a problem on heterogeneous configurations.

Signed-off-by: Jan Beulich <jbeulich@novell.com>