drm/i915: don't allocate the ring in stolen if we lack aperture

Since we have no way access it from the CPU. For such cases just
fallback to internal objects.

Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20191029095856.25431-6-matthew.auld@intel.com