thermal/drivers/devfreq_cooling: Fix the build when !ENERGY_MODEL

Prevent build failure if the option CONFIG_ENERGY_MODEL is not set. The
devfreq cooling is able to operate without the Energy Model.
Don't use dev->em_pd directly and use local pointer.

Fixes: 615510fe ("thermal: devfreq_cooling: remove old power model and use EM")
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Lukasz Luba <lukasz.luba@arm.com>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Link: https://lore.kernel.org/r/20201215154221.8828-1-lukasz.luba@arm.com