i915/guc: Get runtime pm in busyness worker only if already active

i915/guc: Get runtime pm in busyness worker only if already active

Ideally the busyness worker should take a gt pm wakeref because the
worker only needs to be active while gt is awake. However, the gt_park
path cancels the worker synchronously and this complicates the flow if
the worker is also running at the same time. The cancel waits for the
worker and when the worker releases the wakeref, that would call gt_park
and would lead to a deadlock.

The resolution is to take the global pm wakeref if runtime pm is already
active. If not, we don't need to update the busyness stats as the stats
would already be updated when the gt was parked.

Note:
- We do not requeue the worker if we cannot take a reference to runtime
  pm since intel_guc_busyness_unpark would requeue the worker in the
  resume path.

- If the gt was parked longer than time taken for GT timestamp to roll
  over, we ignore those rollovers since we don't care about tracking the
  exact GT time. We only care about roll overs when the gt is active and
  running workloads.

- There is a window of time between gt_park and runtime suspend, where
  the worker may run. This is acceptable since the worker will not find
  any new data to update busyness.

v2: (Daniele)
- Edit commit message and code comment
- Use runtime pm in the worker
- Put runtime pm after enabling the worker
- Use Link tag and add Fixes tag

v3: (Daniele)
- Reword commit and comments and add details

Link: https://gitlab.freedesktop.org/drm/intel/-/issues/7077
Fixes: 77cdd054dd2c ("drm/i915/pmu: Connect engine busyness stats from GuC to pmu")
Signed-off-by: Umesh Nerlige Ramappa <umesh.nerlige.ramappa@intel.com>
Reviewed-by: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230925192117.2497058-1-umesh.nerlige.ramappa@intel.com