omxvideoenc: fix vertical padding in NV16 formats
omxvideoenc: fix vertical padding in NV16 formats

My previous patch to calculate the vertical padding was always halfing
the height of the chroma plane which is incorrect for NV16 formats.

https://bugzilla.gnome.org/show_bug.cgi?id=796749

GStreamer/gst-omx,GStreamer/gst-omx,GStreamer/gst-omx