mali_kbase: platform: power: Prevent races on power down

The power down sequence on the GPU can be triggered by more than
one event. To avoid races when these events are quasi-synchronous
we add a lock to protect the critical section.

Bug: 196269975
Bug: 196300136
Signed-off-by: Sidath Senanayake <sidaths@google.com>
Change-Id: I8f70003dea8d47fc588c18473439bf113b18f3a0