OPP: Remove genpd_virt_dev_lock

All the config operations for OPP tables share common code paths now and
none of the other ones have such protection in place. Either all should
have it or none.

The understanding here is that user won't clear the OPP configs while
still using them and so such a case won't happen. We can always come
back and use a wider lock for all resource types if required.

Also fix the error on failing to allocate memory.

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