Revert "st/pbo: use cso_set_vertex_buffers_and_elements() for st_pbo_draw"

cso_set_vertex_buffers_and_elements shouldn't be used between cso_save and cso_restore because it can switch for cso_context to u_vbuf and vice versa. Also, ẗhe preferred order of calls for future VB+draw merging would be set_vertex_elements followed by set_vertex_buffers.

This reverts commit 33e117ed.

Fixes: 33e117ed Closes: #4674 (closed)