msm: kgsl: Correctly unregister OPP notifier

Use dev_pm_opp_unregister_notifier instead of
dev_pm_opp_register_notifier to unregister OPP
notifier correctly in kgsl_pwrscale_close.

Change-Id: I4bd2df50270cbaf926f985dce58f558d122145a2
Signed-off-by: Deepak Kumar <dkumar@codeaurora.org>