drm/mediatek: ovl: Fix XRGB format breakage for blend_modes unsupported SoCs

OVL_CON_AEN is for alpha blending enable.
For the SoC that is supported the blend_modes, OVL_CON_AEN will always
enabled to use constant alpha and then use the ignore_pixel_alpha bit
to do the alpha blending for XRGB8888 format.
Note that ignore pixel alpha bit is not supported if the SoC is not
supported the blend_modes.
So it will break the original setting of XRGB8888 format for the
blend_modes unsupported SoCs, such as MT8173.

To fix the downgrade issue, enable alpha blending only when a valid
blend_mode or has_alpha is set.

Fixes: bc46eb5d ("drm/mediatek: Support DRM plane alpha in OVL")
Signed-off-by: Jason-JH.Lin <jason-jh.lin@mediatek.com>
Reviewed-by: CK Hu <ck.hu@mediatek.com>
Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Link: https://patchwork.kernel.org/project/dri-devel/patch/20241009034646.13143-2-jason-jh.lin@mediatek.com/


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