v4l2: videodec: Prefer acquired caps over anything downstream

As we don't have anything smart in the fixation process, we may endup with a format that has a lower bitdepth, even if downstream can handle higher depth. it is notably the case when negotiating with deinterlace, which places is non-passthrough caps before its passthrough one. This makes the generic fixation prefer the formats natively supported by deinterlace element over the HW 10bit format. As some HW can downscale 10bit to 8bit, this can break 10bit decoding.

This replaces !4295 (closed), would be nice @hq if you could test this for me, as I'm not currently up and running for that. Please enable the leak tracer, just to make sure I didn't leak a caps here.