drm/i915: Assert all sg are initialised in fake_dma_object for selftests

Double check that we allocated the right amount of scatterlist elements
for our obj->size.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Matthew Auld <matthew.william.auld@gmail.com>
Reviewed-by: Matthew Auld <matthew.william.auld@gmail.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170225181122.4788-1-chris@chris-wilson.co.uk