drm/i915: prefer i915_gem_object_has_pages()

We have an existing helper for testing obj->mm.pages, so use it.
Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20171218103855.25274-1-matthew.auld@intel.comReviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>

drivers/gpu/drm/i915/i915_gem.c

@@ -2596,7 +2596,7 @@ static int ____i915_gem_object_get_pages(struct drm_i915_gem_object *obj)
 	}
 
 	err = obj->ops->get_pages(obj);
-	GEM_BUG_ON(!err && IS_ERR_OR_NULL(obj->mm.pages));
+	GEM_BUG_ON(!err && !i915_gem_object_has_pages(obj));
 
 	return err;
 }

drivers/gpu/drm/i915/selftests/huge_pages.c

@@ -1637,7 +1637,7 @@ static int igt_shrink_thp(void *arg)
 	 * shmem to truncate our pages.
 	 */
 	i915_gem_shrink_all(i915);
-	if (!IS_ERR_OR_NULL(obj->mm.pages)) {
+	if (i915_gem_object_has_pages(obj)) {
 		pr_err("shrink-all didn't truncate the pages\n");
 		err = -EINVAL;
 		goto out_close;