horizontal_backporch_byte should be hbp * bpp - hbp extra bytes.
So remove the wrong subtraction 10.

Fixes: 7a5bc4e2 ("drm/mediatek: change the dsi phytiming calculate method")
Signed-off-by: Jitao Shi <jitao.shi@mediatek.com>
Signed-off-by: Chun-Kuang Hu <chunkuang.hu@kernel.org>