PM / Domains: Validate cases of a non-bound driver in genpd governor

Recently genpd removed the requirement of a having a driver bound for its
attached devices to allow genpd to power off. That change should also have
removed a corresponding validation in the governor, let's correct that.

Fixes: 298cd0f0 (PM / Domains: Remove dev->driver check for runtime)
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>