devfreq: rk3399_dmc: Don't use OPP structures outside of RCU locks

The OPP structures are abused to the best here, without understanding
how the OPP core and RCU locks work.

In short, the OPP pointer saved in 'rk3399_dmcfreq' can become invalid
under your nose, as the OPP core may free it.

Fix various abuses around OPP structures and calls.

Reviewed-by: Chanwoo Choi <cw00.choi@samsung.com>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>