sched/energy: remove rcu lock around dev_pm_opp_find_freq_floor

Commit 052c6f19 ("PM / OPP: Move away from RCU locking")
removed need for RCU locking and introduced mutex lock, so, remove
rcu locking around dev_pm_opp_find_freq_floor call to avoid
calling sleeping function inside atomic context.

Change-Id: Id50bab9a6b9b3debb85af57bd16af22ac29e1398
Signed-off-by: Satya Durga Srinivasu Prabhala <satyap@codeaurora.org>