PM / Domains: Deal with multiple states but no governor in genpd

A caller of pm_genpd_init() that provides some states for the genpd via the
->states pointer in the struct generic_pm_domain, should also provide a
governor. This because it's the job of the governor to pick a state that
satisfies the constraints.

Therefore, let's print a warning to inform the user about such bogus
configuration and avoid to bail out, by instead picking the shallowest
state before genpd invokes the ->power_off() callback.

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