drm/mediatek: dsi: Correct calculation formula of PHY Timing

This patch correct calculation formula of PHY timing.
The spec define HS-PREPARE should be from 40ns+4*UI(44ns) to
85ns+6*UI(91ns). But current duration is 88ns and is near the boundary.
So this patch make the duration to 64ns so it is near the safe range.

Signed-off-by: Shuijing Li <shuijing.li@mediatek.com>
Reviewed-by: CK Hu <ck.hu@mediatek.com>
Link: https://patchwork.kernel.org/project/dri-devel/patch/20240412031208.30688-1-shuijing.li@mediatek.com/
Signed-off-by: Chun-Kuang Hu <chunkuang.hu@kernel.org>