v4l2: don't add the same interlace mode twice
v4l2: don't add the same interlace mode twice

Some drivers modify the interlace mode to progressive, no matter what
input you give them, make sure that we don't add the same interlace mode
twice.

vatavuserban/gst-plugins-good,rawoul/gst-plugins-good,stfl/gst-plugins-good,GrokImageCompression/gst-plugins-good,kittee/gst-plugins-good,GrokImageCompression/gst-plugins-good,pexip/gst-plugins-good,sebras/gst-plugins-good,shelsonjava/gst-plugins-good,rawoul/gst-plugins-good,StreamUtils/gst-plugins-good,StreamUtils/gst-plugins-good,hizukiayaka/gst-plugins-good,froggatt/gst-plugins-good-m,Kurento/gst-plugins-good,ikonst/gst-plugins-good,chamois94/gst-plugins-good,GStreamer/gst-plugins-good,kittee/gst-plugins-good,pexip/gst-plugins-good,pexip/gst-plugins-good,GStreamer/gst-plugins-good,Kurento/gst-plugins-good,hizukiayaka/gst-plugins-good,hizukiayaka/gst-plugins-good,chamois94/gst-plugins-good,vatavuserban/gst-plugins-good,GrokImageCompression/gst-plugins-good,sebras/gst-plugins-good,chamois94/gst-plugins-good,GStreamer/gst-plugins-good,kittee/gst-plugins-good,pexip/gst-plugins-good,rawoul/gst-plugins-good,hizukiayaka/gst-plugins-good,StreamUtils/gst-plugins-good,rawoul/gst-plugins-good,ikonst/gst-plugins-good,vatavuserban/gst-plugins-good,Kurento/gst-plugins-good,pexip/gst-plugins-good,froggatt/gst-plugins-good-m,shelsonjava/gst-plugins-good,froggatt/gst-plugins-good-m,shelsonjava/gst-plugins-good,ikonst/gst-plugins-good,sebras/gst-plugins-good,StreamUtils/gst-plugins-good,GStreamer/gst-plugins-good,froggatt/gst-plugins-good-m,stfl/gst-plugins-good,stfl/gst-plugins-good,ikonst/gst-plugins-good,Kurento/gst-plugins-good,chamois94/gst-plugins-good,stfl/gst-plugins-good,sebras/gst-plugins-good,kittee/gst-plugins-good,shelsonjava/gst-plugins-good,Kurento/gst-plugins-good,vatavuserban/gst-plugins-good,GrokImageCompression/gst-plugins-good