mali_kbase: pm: Calculate metrics only when initialized

It is possible in some cases for power management metrics
calculations to be started before the power management
infrastructure has been initialized. This change tracks
that initialization to ensure that we don't hit this issue.

Bug: 171680119
Bug: 170498705
Signed-off-by: Sidath Senanayake <sidaths@google.com>
Change-Id: Ie02eaaf15814125332a4e1419db90b8394656c63