drm/mediatek: reduce the hbp and hfp for phy timing

There are some extra data transfer in dsi.
ex. LPX, hs_prepare, hs_zero, hs_exit and the sof/eof of dsi packet.
This signal will enlarge the line time. So the real frame on dsi bus
will be lower than calc by video timing.

So dsi driver reduces the hbp and hfp to keep the line time.

Fixes: 7a5bc4e2 ("drm/mediatek: change the dsi phytiming calculate method")
Signed-off-by: Jitao Shi <jitao.shi@mediatek.com>
Tested-by: Hsin-Yi Wang <hsinyi@chromium.org>
Tested-by: Enric Balletbo i Serra <enric.balletbo@collabora.com>
Signed-off-by: CK Hu <ck.hu@mediatek.com>