mesa/st: Avoid extra references in the feedback draw function scope.

mesa/st: Avoid extra references in the feedback draw function scope.

The change removes the reference that is held on the entries of the
vbuffers[] array. The new code does not do that anymore as following
the code into draw_set_vertex_buffers() the draw context holds an
other reference as long as it is reset down the function again.
So it should be already by that argument save to remove that
additional reference count.

Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Signed-off-by: Mathias Fröhlich <Mathias.Froehlich@web.de>