nv50: don't flush vertex arrays when index buffer changes

The index buffer is fed in inline over a pushbuf. It's not related to
vertices or any caching that might be done on them.

Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
Cc: mesa-stable@lists.freedesktop.org

src/gallium/drivers/nouveau/nv50/nv50_vbo.c

          nv50->base.vbo_dirty = true;
    }
 
-   if (!nv50->base.vbo_dirty && nv50->idxbuf.buffer &&
-       nv50->idxbuf.buffer->flags & PIPE_RESOURCE_FLAG_MAP_COHERENT)
-      nv50->base.vbo_dirty = true;
-
    if (nv50->base.vbo_dirty) {
       BEGIN_NV04(push, NV50_3D(VERTEX_ARRAY_FLUSH), 1);
       PUSH_DATA (push, 0);