Revert "drm/i915: set dummy page for stolen objects"

Since for_each_sg_page supports already memory w/o backing pages we can
revert the corresponding workaround.

This reverts commit 5bd4687e.

Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>