Previously, the same struct was used for linux-dmabuf-v1 params
and buffer. This made the whole logic a little bit awkward, because
a wlr_dmabuf_v1_buffer could either be still being constructed, or
be a complete buffer.
Introduce a separate wlr_linux_buffer_params_v1 struct for buffer
params still being constructed. Once the params are complete (ie.
once the create request is sent), the params struct is destroyed
and the buffer struct is created.
This will help with [1] as well.
[1]: https://github.com/swaywm/wlroots/issues/2664
