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