drm/i915: Be defensive when cleaning up i915_gem_internal pages

If we abort the i915_gem_internal get_pages, we mark the failing sg as
the last. However, that means we iterate upto and including the failing
sg element and results in us trying to free the unallocated sg_page().
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: http://patchwork.freedesktop.org/patch/msgid/20170131104630.3074-1-chris@chris-wilson.co.ukReviewed-by: Matthew Auld <matthew.auld@intel.com>

drivers/gpu/drm/i915/i915_gem_internal.c

@@ -35,8 +35,10 @@ static void internal_free_pages(struct sg_table *st)
 {
 	struct scatterlist *sg;
 
-	for (sg = st->sgl; sg; sg = __sg_next(sg))
-		__free_pages(sg_page(sg), get_order(sg->length));
+	for (sg = st->sgl; sg; sg = __sg_next(sg)) {
+		if (sg_page(sg))
+			__free_pages(sg_page(sg), get_order(sg->length));
+	}
 
 	sg_free_table(st);
 	kfree(st);
@@ -116,6 +118,7 @@ i915_gem_object_get_pages_internal(struct drm_i915_gem_object *obj)
 	return st;
 
 err:
+	sg_set_page(sg, NULL, 0, 0);
 	sg_mark_end(sg);
 	internal_free_pages(st);
 	return ERR_PTR(-ENOMEM);