opp: Reuse the enabled flag in !target_freq path

The OPP core needs to track if the resources of devices are
enabled/configured or not, as it disables the resources when target_freq
is set to 0.

Handle that with the new enabled flag and remove otherwise complex
conditional statements.

Tested-by: Rajendra Nayak <rnayak@codeaurora.org>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>