drm/i915: Avoid round-trip via i915 in intel_gt_park

Both in the container_of and getting to gt->awake there is no need to go
via i915 since both the wakeref and awake are members of gt.

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: Andi Shyti <andi.shyti@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20190910143823.10686-4-tvrtko.ursulin@linux.intel.com