drm/tegra: dc: Using NULL instead of plain integer

Fixes the following sparse warnings:

drivers/gpu/drm/tegra/dc.c:2181:69: warning:
  Using plain integer as NULL pointer

Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>