drm/panfrost: dev_pm_opp_put_*() accepts NULL argument

The dev_pm_opp_put_*() APIs now accepts a NULL opp_table pointer and so
there is no need for us to carry the extra check. Drop them.

Reviewed-by: Steven Price <steven.price@arm.com>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>