drm/mediatek: Use NULL instead of 0 for NULL pointer

Use NULL for NULL pointer to fix the following sparse warning:
drivers/gpu/drm/mediatek/mtk_drm_gem.c:265:27: sparse: warning: Using plain integer as NULL pointer

Fixes: 3df64d7b ("drm/mediatek: Implement gem prime vmap/vunmap function")
Signed-off-by: Miles Chen <miles.chen@mediatek.com>
Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Link: https://patchwork.kernel.org/project/linux-mediatek/patch/20230111024443.24559-1-miles.chen@mediatek.com/


Signed-off-by: Chun-Kuang Hu <chunkuang.hu@kernel.org>