msm: kgsl: Put OPP reference count

dev_pm_opp_find_freq_* and devfreq_recommended_opp APIs
takes a reference count of the returned OPP. Put this
reference count after using OPP. Also, drop RCU read
locks as it is guaranteed that OPP wouldn't get freed
because of the reference count.

Change-Id: I2da8d5a63a7071e8670fa3152003e570ceeba76d
Signed-off-by: Deepak Kumar <dkumar@codeaurora.org>