cpufreq: amd_pstate: fix wrong lowest perf fetch

Fix the wrong lowest perf value reading which is used for new
des_perf calculation by governor requested, the incorrect min_perf will
get incorrect des_perf to be set , that will cause the system frequency
changing unexpectedly.

Reviewed-by: Huang Rui <ray.huang@amd.com>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Perry Yuan <Perry.Yuan@amd.com>
Signed-off-by: Su Jinzhou <jinzhou.su@amd.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>