drm/i915/perf: Track the rpm wakeref

Keep track of our wakeref used to keep the device awake so we can catch
any leak.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190114142129.24398-7-chris@chris-wilson.co.uk