OPP: No need to defer probe from _opp_attach_genpd()

When the new interface for attaching genpd's via the OPP core was added,
it was possible for required_opp_count to be zero, but not anymore.

Remove the unused check.

Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Reviewed-by: Ulf Hansson <ulf.hansson@linaro.org>