Currently `nvim -u NORC --cmd "set display-=msgsep"` will still allocate the
message grid and remove it just afterwards. While inefficient, we must
make sure update_screen() re-validates the default_grid completely when
this happens.
Fix some invalid logic: don't reallocate msg_grid on resize when the grid is not
used.
Elide a too early ui_flush() on startup, which caused an invalid cursor
position to be used.