st/nine: Rework update_vertex_buffers

st/nine: Rework update_vertex_buffers

Previous code was trying to optimise to call set_vertex_buffers on
big packets, and thus avoids as many calls as possible.

However in practice doing so won't be faster (drivers implement
set_vertex_buffers by a loop over the buffers we want to bind)

When we want to unbind a buffer, we were calling set_vertex_buffers
on a buffer with vtxbuf->buffer = NULL. It works on some drivers,
but not on all of them, because it isn't in Gallium spec.
This patch fixes that.

Reviewed-by: Ilia Mirkin <imirkin@alum.mit.edu>
Signed-off-by: Axel Davy <axel.davy@ens.fr>