v4l2object: Validate field in S/TRY_FMT
v4l2object: Validate field in S/TRY_FMT

This is in preparation from removing the slow TRY_FMT probes for
interlacing. As we won't have tried that interlace-mode already
we need to validate that the driver isn't refusing it.

https://bugzilla.gnome.org/show_bug.cgi?id=785156

StreamUtils/gst-plugins-good,StreamUtils/gst-plugins-good,StreamUtils/gst-plugins-good,GrokImageCompression/gst-plugins-good,GrokImageCompression/gst-plugins-good,pexip/gst-plugins-good,pexip/gst-plugins-good,pexip/gst-plugins-good,GrokImageCompression/gst-plugins-good,GStreamer/gst-plugins-good,GStreamer/gst-plugins-good,GStreamer/gst-plugins-good,pexip/gst-plugins-good,GStreamer/gst-plugins-good,StreamUtils/gst-plugins-good,GrokImageCompression/gst-plugins-good,pexip/gst-plugins-good