draw: use the vertex size

Instead of using the magical 4 use the above computed
vertex size. Doesn't change the behavior, just makes the code
a bit cleaner.

Signed-off-by: Zack Rusin <zackr@vmware.com>
Reviewed-by: Roland Scheidegger <sroland@vmware.com>
Reviewed-by: Jose Fonseca <jfonseca@vmware.com>

src/gallium/auxiliary/draw/draw_pipe_vbuf.c

@@ -250,7 +250,7 @@ vbuf_start_prim( struct vbuf_stage *vbuf, uint prim )
    }
 
    hw_key.nr_elements = vbuf->vinfo->num_attribs;
-   hw_key.output_stride = vbuf->vinfo->size * 4;
+   hw_key.output_stride = vbuf->vertex_size;
 
    /* Don't bother with caching at this stage:
     */