cpupower: Do not analyse offlined cpus

Use sysfs_is_cpu_online(cpu) instead of cpufreq_cpu_exists(cpu) to detect offlined cpus.

Re-arrange printfs slightly to have a consistent output even if you have multiple CPUs
as output and even if offlined cores are in between.

Signed-off-by: Thomas Renninger <trenn@suse.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>