mpeg2enc: fix interlace-mode detection

Previously, the code was always assuming progressive input,
fix this by looking at the caps.

Part-of: <gstreamer/gst-plugins-bad!2500>
44 jobs for !2500 with backport-mr-2455-into-1.18-mpeg2enc-interlace-mode in 42 minutes and 59 seconds (queued for 24 seconds)
merge request