cpufreq/amd-pstate: Drop min and max cached frequencies

Use the perf_to_freq helpers to calculate this on the fly.
As the members are no longer cached add an extra check into
amd_pstate_epp_update_limit() to avoid unnecessary calls in
amd_pstate_update_min_max_limit().

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>