fix ticket: 8932

For poc 2, we have tile boundary at x = 640.
When we predict cu(640,912),the top left pixel is not avaliable to the cu.
So, we can not check it's intra or not. We need set top[-1] = top[0] directly.
see 8.4.4.2.1 for details
Signed-off-by: Xu Guangxin <oddstone@gmail.com>
Signed-off-by: Linjie Fu <linjie.justin.fu@gmail.com>