PM / Domains: Simplify genpd_lookup_dev()

genpd_lookup_dev(), is a bit unnecessary heavy, as it walks the gpd_list to
try to find a valid PM domain corresponding to the device's attached genpd.

Instead of walking the gpd_list, let's use the fact that a genpd always has
the ->runtime_suspend() callback assigned to the genpd_runtime_suspend()
function.

While changing this, let's take the opportunity to also rename
genpd_lookup_dev(), into dev_to_genpd_safe() to better reflect its purpose.

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