drm/damage-helper: Use NULL instead of 0

The 'clips' member is a pointer, so assign NULL instead of 0.

This fixes the following sparse warning:

drivers/gpu/drm/drm_damage_helper.c:289:31: warning: Using plain integer as NULL pointer
Signed-off-by: Fabio Estevam <festevam@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20190527183714.11168-1-festevam@gmail.com

drivers/gpu/drm/drm_damage_helper.c

@@ -286,7 +286,7 @@ drm_atomic_helper_damage_iter_init(struct drm_atomic_helper_damage_iter *iter,
 	iter->plane_src.y2 = (state->src.y2 >> 16) + !!(state->src.y2 & 0xFFFF);
 
 	if (!iter->clips || !drm_rect_equals(&state->src, &old_state->src)) {
-		iter->clips = 0;
+		iter->clips = NULL;
 		iter->num_clips = 0;
 		iter->full_update = true;
 	}