clients/simple-dmabuf-feedback: Support multi-tranche feedbacks

Compositors may choose to send multiple scanout or non-scanout
tranches. So instead of assuming that the first respective tranche
contains the format/modifier we're looking for, check all tranches.

While on it, make sure that in case a compositor sends scanout
tranches on the initial feedback, `pick_format_from_scanout_tranche()`
does not unintentionally pick `INITIAL_BUFFER_FORMAT`.

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