mali_kbase: pm: Fix APC race condition

It was possible for the APC power off work queue to use the
same work struct for two different power off requests if two
consecutive APC requests were issued in quick succession with
sufficiently short duration requests.

This change ensures that the power off work queue is only used
once by ensuring that all active hrtimer callbacks complete
before the next power off work can be set up.

The change also moves power on and power off work struct
initialization to APC init as they are only ever used with a
single worker struct.

Bug: 190341904
Signed-off-by: Sidath Senanayake <sidaths@google.com>
Change-Id: Iaf08d9a1ff0a1a824d00a505ce71880402ed1552