drm/mediatek: Fix XRGB setting error in Mixer

Although the alpha channel in XRGB formats can be ignored, ALPHA_CON
must be configured accordingly when using XRGB formats or it will still
affects CRC generation.

Fixes: d886c000 ("drm/mediatek: Add ETHDR support for MT8195")
Reviewed-by: CK Hu <ck.hu@mediatek.com>
Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Signed-off-by: Hsiao Chien Sung <shawn.sung@mediatek.com>
Link: https://patchwork.kernel.org/project/dri-devel/patch/20240620-igt-v3-4-a9d62d2e2c7e@mediatek.com/


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