linux-dmabuf: Add support for protocol version 5

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.

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Draft pending a wayland-protocols release.