drm/mediatek: Support "Pre-multiplied" blending in OVL

Support "Pre-multiplied" alpha blending mode on in OVL.
Before this patch, only the "coverage" mode is supported.

As whether OVL_CON_CLRFMT_MAN bit is enabled, (3 << 12)
means different formats in the datasheet. To prevent
misunderstandings going forward, instead of reusing
OVL_CON_CLRFMT_RGBA8888, we intetionally defined
OVL_CON_CLRFMT_PARGB8888 with bit operation again.

Reviewed-by: CK Hu <ck.hu@mediatek.com>
Signed-off-by: Hsiao Chien Sung <shawn.sung@mediatek.com>
Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Link: https://patchwork.kernel.org/project/dri-devel/patch/20240717-alpha-blending-v4-3-4b1c806c0749@mediatek.com/


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