drm/atomic: fix warning of unused variable

Fix the `unused-but-set-variable` warning as how other iteration
wrappers do.

Link: https://lore.kernel.org/all/202206071049.pofHsRih-lkp@intel.com/
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: GONG, Ruiqi <gongruiqi1@huawei.com>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Link: https://patchwork.freedesktop.org/patch/msgid/20220607110848.941486-1-gongruiqi1@huawei.com

Author: GONG, Ruiqi

include/drm/drm_atomic.h

@@ -1022,6 +1022,7 @@ void drm_state_dump(struct drm_device *dev, struct drm_printer *p);
 	for ((__i) = 0; \
 	     (__i) < (__state)->num_private_objs && \
 		     ((obj) = (__state)->private_objs[__i].ptr, \
+		      (void)(obj) /* Only to avoid unused-but-set-variable warning */, \
 		      (new_obj_state) = (__state)->private_objs[__i].new_state, 1); \
 	     (__i)++)