drm/i915: Add a test that we terminate the trimmed sgtable as expected

In commit 0c40ce13

 ("drm/i915: Trim the object sg table"), we expect
to copy exactly orig_st->nents across and allocate the table thusly.
The copy loop should therefore end with the new_sg being NULL.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20161219124346.550-2-chris@chris-wilson.co.uk
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>