r300g: mark vertex arrays as dirty after a buffer_offset change

We shouldn't hit this bug in theory.

NOTE: This is a candidate for the 7.10 branch.

src/gallium/drivers/r300/r300_screen_buffer.c

@@ -90,6 +90,7 @@ void r300_upload_user_buffers(struct r300_context *r300)
                           &vb->buffer_offset, &vb->buffer);
 
             r300->validate_buffers = TRUE;
+            r300->aos_dirty = TRUE;
         }
     }
 }