drm/i915/selftests: Mark the mock ring->vma as being in the GGTT

The ringbuffer's vma is expected to be in the GGTT and that is now being
checked, so make sure the mocked ring declares it so.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200215205927.4170144-1-chris@chris-wilson.co.uk