zink: re-transform gl_Position for gs input

we've transformed this in the vertex output, so we need to undo that here

ideally we'd only be performing this transform once, but that's going to get
complicated later with the halfz extension which requires shader keys on top
of this, so we can get around to simplifying things at that stage

Reviewed-by: Erik Faye-Lund <erik.faye-lund@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/7139>
1 file changed