drm/amd/display: fix gcc set but not used warning of variable 'old_plane_state'

define a new macro for_each_new_plane_in_state_reverse to replace
for_each_oldnew_plane_in_state_reverse, so that the unused variable
'old_plane_state' can be removed.

Fix gcc warning:
drivers/gpu/drm/amd/amdgpu/../display/amdgpu_dm/amdgpu_dm.c:10066:26: warning:
 variable ‘old_plane_state’ set but not used [-Wunused-but-set-variable]
Signed-off-by: Yu Kuai <yukuai3@huawei.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c

@@ -10055,11 +10055,11 @@ static int validate_overlay(struct drm_atomic_state *state)
 {
 	int i;
 	struct drm_plane *plane;
-	struct drm_plane_state *old_plane_state, *new_plane_state;
+	struct drm_plane_state *new_plane_state;
 	struct drm_plane_state *primary_state, *cursor_state, *overlay_state = NULL;
 
 	/* Check if primary plane is contained inside overlay */
-	for_each_oldnew_plane_in_state_reverse(state, plane, old_plane_state, new_plane_state, i) {
+	for_each_new_plane_in_state_reverse(state, plane, new_plane_state, i) {
 		if (plane->type == DRM_PLANE_TYPE_OVERLAY) {
 			if (drm_atomic_plane_disabling(plane->state, new_plane_state))
 				return 0;

include/drm/drm_atomic.h

@@ -895,6 +895,18 @@ void drm_state_dump(struct drm_device *dev, struct drm_printer *p);
 			      (old_plane_state) = (__state)->planes[__i].old_state,\
 			      (new_plane_state) = (__state)->planes[__i].new_state, 1))
 
+/**
+ * for_each_new_plane_in_state_reverse - other than only tracking new state,
+ * it's the same as for_each_oldnew_plane_in_state_reverse
+ */
+#define for_each_new_plane_in_state_reverse(__state, plane, new_plane_state, __i) \
+	for ((__i) = ((__state)->dev->mode_config.num_total_plane - 1);	\
+	     (__i) >= 0;						\
+	     (__i)--)							\
+		for_each_if ((__state)->planes[__i].ptr &&		\
+			     ((plane) = (__state)->planes[__i].ptr,	\
+			      (new_plane_state) = (__state)->planes[__i].new_state, 1))
+
 /**
  * for_each_old_plane_in_state - iterate over all planes in an atomic update
  * @__state: &struct drm_atomic_state pointer