tools/power turbostat: Ignore pkg_cstate_limit when it is not available

When running in no-msr mode, the pkg_cstate_limit is not populated, thus
we use perf to determine if given pcstate counter is present on the
platform.

Signed-off-by: Patryk Wlazlyn <patryk.wlazlyn@linux.intel.com>
Signed-off-by: Len Brown <len.brown@intel.com>