drm: add helper to walk a dma scatter list a page at a time

Add a helper to walk through a scatter list a page at a time. Needed by
upcoming patches fixing the scatter list walking logic in the i915 driver.

Signed-off-by: Imre Deak <imre.deak@intel.com>
drm: handle compact dma scatter lists in drm_clflush_sg()

So far the assumption was that each scatter list entry contains a single
page. This might not hold in the future, when we'll introduce compact
scatter lists, so prepare for this here.

Reference: http://www.spinics.net/lists/dri-devel/msg33917.html
Signed-off-by: Imre Deak <imre.deak@intel.com>
drm/i915: handle walking compact dma scatter lists

So far the assumption was that each dma scatter list entry contains only
a single page. This might not hold in the future, when we'll introduce
compact scatter lists, so prepare for this everywhere in the i915 code
where we walk such a list.

We'll fix the place _creating_ these lists separately in the next patch
to help the reviewing/bisectability.

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

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