mali_kbase: platform: dvfs: Update QOS on clock down

Due to a bug, QOS state wasn't being re-evaluated when the
GPU clocked down to a lower operating point. This change
fixes this by only updating the current level once all QOS
evaluations have completed.

This change also tightens the duration for which the GPU's
PM Domain Access Lock is held.

Signed-off-by: Sidath Senanayake <sidaths@google.com>
Change-Id: I55f646e0ad04473ec7d553b82f4fad9836f7cff0