drm/mediatek: dsi: Modify horizontal front/back porch byte formula

In the patch to be fixed, horizontal_backporch_byte become too large
for some panel, so roll back that patch. For small hfp or hbp panel,
using vm->hfront_porch + vm->hback_porch to calculate
horizontal_backporch_byte would make it negtive, so
use horizontal_backporch_byte itself to make it positive.

Fixes: 35bf948f ("drm/mediatek: dsi: Fix scrolling of panel with small hfp or hbp")

Signed-off-by: CK Hu <ck.hu@mediatek.com>
Signed-off-by: Chun-Kuang Hu <chunkuang.hu@kernel.org>
Tested-by: Bilal Wasim <bilal.wasim@imgtec.com>