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>

drivers/gpu/drm/mediatek/mtk_ethdr.c

@@ -50,7 +50,6 @@
 
 #define MIXER_INX_MODE_BYPASS			0
 #define MIXER_INX_MODE_EVEN_EXTEND		1
-#define DEFAULT_9BIT_ALPHA			0x100
 #define	MIXER_ALPHA_AEN				BIT(8)
 #define	MIXER_ALPHA				0xff
 #define ETHDR_CLK_NUM				13
@@ -169,7 +168,7 @@ void mtk_ethdr_layer_config(struct device *dev, unsigned int idx,
 		alpha_con = MIXER_ALPHA_AEN | MIXER_ALPHA;
 
 	mtk_mmsys_mixer_in_config(priv->mmsys_dev, idx + 1, alpha_con ? false : true,
-				  DEFAULT_9BIT_ALPHA,
+				  MIXER_ALPHA,
 				  pending->x & 1 ? MIXER_INX_MODE_EVEN_EXTEND :
 				  MIXER_INX_MODE_BYPASS, align_width / 2 - 1, cmdq_pkt);