panfrost: Rename depth bias fields

panfrost: Rename depth bias fields

Make it clear that the distinction is the facingness of the primitives
the depth bias applies to.

Signed-off-by: Alyssa Rosenzweig <alyssa@collabora.com>
Reviewed-by: Boris Brezillon <boris.brezillon@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/13509>