clients/simple-dmabuf-feedback: Add fallback print method for unknown formats

Using `pixel_format_get_info()` can result in formats being
reported as `UNKNOWN` when used on compositors other than Weston.

As `weston-simple-dmabuf-feedback` somewhat succeeds `wayland-info`
as tool for `zwp_linux_dmabuf_v1` debugging from version 4 on, copy
the approach from the later for these cases.

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