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>

drivers/gpu/drm/i915/i915_gem_stolen.c

@@ -222,8 +222,8 @@ i915_pages_create_for_stolen(struct drm_device *dev,
 	}
 
 	sg = st->sgl;
-	sg->offset = offset;
-	sg->length = size;
+	/* we set the dummy page here only to make for_each_sg_page work */
+	sg_set_page(sg, dev_priv->gtt.scratch_page, size, offset);
 
 	sg_dma_address(sg) = (dma_addr_t)dev_priv->mm.stolen_base + offset;
 	sg_dma_len(sg) = size;