cpufreq/amd-pstate-ut: Run on all of the correct CPUs

If a CPU is missing a policy or one has been offlined then the unit test
is skipped for the rest of the CPUs on the system.

Instead; iterate online CPUs and skip any missing policies to allow
continuing to test the rest of them.

Reviewed-by: Gautham R. Shenoy <gautham.shenoy@amd.com>
Reviewed-by: Dhananjay Ugwekar <dhananjay.ugwekar@amd.com>
Signed-off-by: Mario Limonciello <mario.limonciello@amd.com>