drm/i915: set dummy page for stolen objects

This is needed since currently sg_for_each_page assumes that we have
a valid page in each sg item. It is only a real problem for
CONFIG_SPARSEMEM where the page is dereferenced, in other cases the
iterator works ok with an invalid page pointer.

We can remove this workaround when we have fixed sg_page_iter to work on
scatterlists without backing pages.

Signed-off-by: Imre Deak <imre.deak@intel.com>