powercap: DTPM: Fix suspend failure and kernel warning

When the ENERGY_MODEL and DTPM_CPU are enabled but actually without
any energy model, at cpu hotplug time, the dead cpuhp callback fails
leading to the warning.

Actually, the check could be simplified and we only do an action if
the dtpm cpu is enabled, otherwise we bail out without error.

Fixes: 7a89d7ea ("powercap/drivers/dtpm: Simplify the dtpm table")
Reported-by: Kenneth R. Crudup <kenny@panix.com>
Tested-by: Kenneth R. Crudup <kenny@panix.com>
Reported-by: kernel test robot <oliver.sang@intel.com>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>