drm/mediatek: ovl: Refine ignore_pixel_alpha comment and placement

drm/mediatek: ovl: Refine ignore_pixel_alpha comment and placement

Refine the comment for ignore_pixel_alpha flag and move it to
if(state->fb) statement to make it less conditional.

Signed-off-by: Jason-JH.Lin <jason-jh.lin@mediatek.com>
Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Reviewed-by: CK Hu <ck.hu@mediatek.com>
Link: https://patchwork.kernel.org/project/dri-devel/patch/20241009034646.13143-3-jason-jh.lin@mediatek.com/
Signed-off-by: Chun-Kuang Hu <chunkuang.hu@kernel.org>