drm/i915: Disallow direct CPU access to stolen pages for relocations

As we cannot access the backing pages behind stolen objects, we should
not attempt to do so for relocations.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20160818161718.27187-15-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/i915_gem_execbuffer.c

@@ -283,6 +283,9 @@ static void eb_destroy(struct eb_vmas *eb)
 
 static inline int use_cpu_reloc(struct drm_i915_gem_object *obj)
 {
+	if (!i915_gem_object_has_struct_page(obj))
+		return false;
+
 	if (DBG_USE_CPU_RELOC)
 		return DBG_USE_CPU_RELOC > 0;