st/mesa: fix crash when using both user and vbo buffers with the same stride

st/mesa: fix crash when using both user and vbo buffers with the same stride

If two buffers had the same stride where one buffer is a user one and
the other is a vbo, it was considered to be one interleaved buffer,
resulting in incorrect rendering and crashes.

This patch makes sure that the interleaved buffer is either user or vbo,
not both.