mali_kbase: dvfs: Implement clock down hysteresis as delayed work

Updates the clock down hysteresis implementation to use
delayed work rather than relying on the the core Mali driver
calling the DVFS handler function.

This also moves the location where hysteresis timer is reset
be when the GPU powers on, rather than in the DVFS worker
thread.

Bug: 171194646
Signed-off-by: Sidath Senanayake <sidaths@google.com>
Change-Id: I5d421ca200aae96fa5dabe2b0c5f670fa94e15ed