deinterlace: greedyh: Use _plane in _packed, fix planar formats

deinterlace: greedyh: Use _plane in _packed, fix planar formats

This greatly reduces code duplication. It also exposed the cause for
planar formats not being properly deinterlaced:

The planar path was missing the initial offset adjustment that the
packed path did to `L2` and `L2P` in the case of an even field, which
caused it to select the wrong weave lines every other field.

Add those offsets in `_plane`.

Fixes: https://gitlab.freedesktop.org/gstreamer/gstreamer/-/issues/1047
Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/1790>