drm/i915/perf: Replace global wakeref tracking with engine-pm

As we now have a specific engine to use OA on, exchange the top-level
runtime-pm wakeref with the engine-pm. This still results in the same
top-level runtime-pm, but with more nuances to keep the engine and its
gt awake.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191011190325.10979-1-chris@chris-wilson.co.uk