drm/i915: Track GT wakeref

Record the wakeref used for keeping the device awake as the GPU is
executing requests and be sure to cancel the tracking upon parking.

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/20190114142129.24398-3-chris@chris-wilson.co.uk