We were previously always querying index 0, and while the number of planes per
buffer will never change, it seems more proper to query the right buffer rather
than always the first one.
This was found while reading strace logs, and wondering why the
V4L2_BUF_FLAG_MAPPED flag was present on all ¬0 indices even though that
happened before VIDIOC_EXPBUF.
Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/5647>