drm/mediatek: Use 8-bit alpha in ETHDR

9-bit alpha (max=0x100) is designed for special HDR related
calculation, which should be disabled by default.
Change the alpha value from 0x100 to 0xff in 8-bit form.

Fixes: d886c000 ("drm/mediatek: Add ETHDR support for MT8195")
Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Reviewed-by: CK Hu <ck.hu@mediatek.com>
Signed-off-by: Hsiao Chien Sung <shawn.sung@mediatek.com>
Link: https://patchwork.kernel.org/project/dri-devel/patch/20240620-igt-v3-2-a9d62d2e2c7e@mediatek.com/


Signed-off-by: Chun-Kuang Hu <chunkuang.hu@kernel.org>