nine: fix incorrect calculation of layer count for 3D textures

Volume textures don't have a concept of "layers"

v1: set last_layer to zero for 3D textures (Axel Davy)

Cc: <mesa-stable@lists.freedesktop.org>
Signed-off-by: Yevhenii Kolesnikov <yevhenii.kolesnikov@globallogic.com>
Reviewed-by: Axel Davy <davyaxel0@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/5808>