It adds the followig paragraph:
```
Starting from version 5, the invalid_format protocol error is sent if
all planes don't use the same modifier.
```

We already assumed this in some places and, most importantly, it's
required by the kernel. Thus alter `dmabuf_attributes.modifier` to make
it clear that different modifiers for multi-planar dmabufs were never
supported.

Signed-off-by: Robert Mader <robert.mader@collabora.com>