drm/mediatek: Fix destination alpha error in OVL

The formula of Coverage alpha blending is:
dst.a = dst.a * (0xff - src.a * SCA / 0xff) / 0xff
      + src.a * SCA / 0xff

dst.a: destination alpha
src.a: pixel alpha
SCA  : plane alpha

When SCA = 0xff, the formula becomes:
dst.a = dst.a * (0xff - src.a) + src.a

This patch is to set the destination alpha (background) to 0xff:
- When dst.a = 0    (before), dst.a = src.a
- When dst.a = 0xff (after) , dst.a = 0xff * (0xff - src.a) + src.a

According to the fomula above:
- When src.a = 0   , dst.a = 0
- When src.a = 0xff, dst.a = 0xff
This two cases are just still correct. But when src.a is
between 0 and 0xff, the difference starts to appear

Fixes: 616443ca ("drm/mediatek: Move cmdq_reg info from struct mtk_ddp_comp to sub driver private data")
Signed-off-by: Hsiao Chien Sung <shawn.sung@mediatek.com>
Link: https://patchwork.kernel.org/project/dri-devel/patch/20240620-igt-v3-5-a9d62d2e2c7e@mediatek.com/


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