drm/i915/selftests: Add a mock i915_vma to the mock_ring

Add a i915_vma to the mock_engine/mock_ring so that the core code can
always assume the presence of ring->vma.

Fixes: 8ccfc20a

 ("drm/i915/gt: Mark ring->vma as active while pinned")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200114160030.2468927-1-chris@chris-wilson.co.uk