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