drm/i915: Store backpointer to intel_gt in the engine

It will come useful in the next patch.

v2:
 * Do mock_engine as well.

v3:
 * And the virtual engine...

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20190621070811.7006-11-tvrtko.ursulin@linux.intel.com