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.
(cherry picked from commit d9b84017e0)

src/gallium/drivers/r300/r300_screen_buffer.c

@@ -119,6 +119,7 @@ int r300_upload_user_buffers(struct r300_context *r300)
             vb->buffer = upload_buffer;
             vb->buffer_offset = upload_offset;
             r300->validate_buffers = TRUE;
+            r300->aos_dirty = TRUE;
         }
     }
     return ret;