drm/i915: create compact dma scatter lists for gem objects

So far we created a sparse dma scatter list for gem objects, where each
scatter list entry represented only a single page. In the future we'll
have to handle compact scatter lists too where each entry can consist of
multiple pages, for example for objects imported through PRIME.

The previous patches have already fixed up all other places where the
i915 driver _walked_ these lists. Here we have the corresponding fix to
_create_ compact lists. It's not a performance or memory footprint
improvement, but it helps to better exercise the new logic.

Reference: http://www.spinics.net/lists/dri-devel/msg33917.html
Signed-off-by: Imre Deak <imre.deak@intel.com>