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