mesa/st: use uint for instance_divisor instead of int

this is confusing

Reviewed-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/12755>

src/mesa/state_tracker/st_atom_array.c

@@ -56,7 +56,7 @@
 static void ALWAYS_INLINE
 init_velement(struct pipe_vertex_element *velements,
               const struct gl_vertex_format *vformat,
-              int src_offset, int instance_divisor,
+              int src_offset, unsigned instance_divisor,
               int vbo_index, bool dual_slot, int idx)
 {
    velements[idx].src_offset = src_offset;