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>
Acked-by: Dave Airlie <airlied@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>