x86: intel_epb: Take CONFIG_PM into account

Commit b9c273ba ("PM / arch: x86: MSR_IA32_ENERGY_PERF_BIAS sysfs
interface") caused kernels built with CONFIG_PM unset to crash on
systems supporting the Performance and Energy Bias Hint (EPB),
because it attempts to add files to sysfs directories that don't
exist on those systems.

Prevent that from happening by taking CONFIG_PM into account so
that the code depending on it is not compiled at all when it is
not set.

Fixes: b9c273ba

 ("PM / arch: x86: MSR_IA32_ENERGY_PERF_BIAS sysfs interface")
Reported-by: Ido Schimmel <idosch@mellanox.com>
Tested-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Acked-by: Ingo Molnar <mingo@kernel.org>