cpufreq/amd-pstate-ut: Don't check for highest perf matching on prefcore

If a system is using preferred cores the highest perf will be inconsistent
as it can change from system events.

Skip the checks for it.

Fixes: e571a5e2 ("cpufreq: amd-pstate: Update amd-pstate preferred core ranking dynamically")
Reviewed-by: Gautham R. Shenoy <gautham.shenoy@amd.com>
Signed-off-by: Mario Limonciello <mario.limonciello@amd.com>