soc: mediatek: Disable 9-bit alpha in ETHDR

When 9-bit alpha is enabled, its value will be converted from 0-255 to
0-256 (255 = not defined). This is designed for special HDR related
calculation, which should be disabled by default, otherwise, alpha
blending will not work correctly.

Signed-off-by: Hsiao Chien Sung <shawn.sung@mediatek.com>
Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Link: https://lore.kernel.org/r/20240620-9bit_alpha-v1-1-13c69daaf29f@mediatek.com


Signed-off-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>