ACPI: CPPC: Add NULL pointer check to cppc_get_perf()

Check cpc_desc against NULL in cppc_get_perf(), so it doesn't crash
down the road if cpc_desc is NULL.

Fixes: 0654cf05 ("ACPI: CPPC: Introduce cppc_get_nominal_perf()")
Reported-by: Kai-Heng Feng <kai.heng.feng@canonical.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Cc: 5.15+ <stable@vger.kernel.org> # 5.15+
Tested-by: Kai-Heng Feng <kai.heng.feng@canonical.com>