drm/doc: Add the "type" plane property to the list of properties

Matt aded this plane property before we had a table giving a summary of
the properties. Add it there.

Cc: Matt Roper <matthew.d.roper@intel.com>
Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>