drm/i915/selftests: Assign the mock_engine->uncore shortcut

Set up the engine->uncore shortcut on mock_engine creation.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191008071121.25088-1-chris@chris-wilson.co.uk