u_vbuf_mgr: restore buffer offsets

u_vbuf_mgr: restore buffer offsets

u_vbuf_upload_buffers modifies the buffer offsets.  If they are not
restored, and any of the vertex formats is not supported natively, the
next u_vbuf_mgr_draw_begin call will translate the vertex buffers with
incorrect buffer offsets.
(cherry picked from commit afc160e1c8f87d0a76e41f1100d528d8ab82ecc4)

Signed-off-by: Marek Olšák <maraeo@gmail.com>