cpufreq: amd-pstate-ut: Modify the function to get the highest_perf value

The previous function amd_get_highest_perf() will be deprecated.
It can only return 166 or 255 by cpuinfo. For platforms that
support preferred core, the value of highest perf can be between
166 and 255. Therefore, it will cause amd-pstate-ut to fail when
run amd_pstate_ut_check_perf().

Signed-off-by: Meng Li <li.meng@amd.com>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>