PM / Domains: Don't check for an existing device when adding a new

When adding a device to a genpd, we no longer need to walk genpd's list
of existing devices to verify it hasn't already been added.

Instead we can now rely on the verification of not allowing existing
generic_pm_domain_data for a device, since that has the same meaning.

Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>