video: Pass component index not plane index

video: Pass component index not plane index

While so far it worked, we are about to introduce a format that break this
assuming. We have a format which consist of NV12 with alpha, and this format
does not have a direct mapping of the component against their plane indexes.

Fix this by using gst_video_format_info_component() introduced in 1.18 for
this purpose.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-plugins-base/-/merge_requests/1151>