mfvideosrc: Don't present interlace-mode in caps while negotating

gst-launch-1.0.exe -v compositor name=comp ! fakesink videotestsrc ! video/x-raw, widht=1280, height=720, framerate=30/1 ! queue ! comp. mfvideosrc ! video/x-raw, framerate=30/1 ! queue ! comp.

Have issue when running this pipeline. Videotestsrc element produces interlace-mode in caps, so compositor forward that mode to mfvideosrc branch. ksvideosrc works fine with this.