OPP: Protect `lazy_opp_tables` list with `opp_table_lock`

The `opp_table_lock` lock is already used to protect the list elsewhere,
use it while adding or removing entries from it.

Reported-by: Stephan Gerhold <stephan.gerhold@kernkonzept.com>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Tested-by: Stephan Gerhold <stephan.gerhold@kernkonzept.com>