OPP: Use _set_opp_level() for single genpd case

OPP: Use _set_opp_level() for single genpd case

There are two genpd (as required-opp) cases that we need to handle,
devices with a single genpd and ones with multiple genpds.

The multiple genpds case is clear, where the OPP core calls
dev_pm_domain_attach_by_name() for them and uses the virtual devices
returned by this helper to call dev_pm_domain_set_performance_state()
later to change the performance state.

The single genpd case however requires special handling as we need to
use the same `dev` structure (instead of a virtual one provided by genpd
core) for setting the performance state via
dev_pm_domain_set_performance_state().

As we move towards more generic code to take care of the required OPPs,
where we will recursively call dev_pm_opp_set_opp() for all the required
OPPs, the above special case becomes a problem.

It doesn't make sense for a device's DT entry to have both "opp-level"
and single "required-opps" entry pointing to a genpd's OPP, as that
would make the OPP core call dev_pm_domain_set_performance_state() for
two different values for the same device structure. And so we can reuse
the 'opp->level" field in such a case and call _set_opp_level() for the
device.

Reviewed-by: Ulf Hansson <ulf.hansson@linaro.org>
Tested-by: Stephan Gerhold <stephan.gerhold@kernkonzept.com>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>