drm/docs: Align layout of optional plane blending properties

Just a bit of drive-by OCD. All the other property docs use enumerations,
for some nice visual consistency. It also neatly highlights the property
name.

Reviewed-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20180219225356.24996-2-daniel.vetter@ffwll.ch