refactor(message): simplify msg_puts_display and use batched grid updates

msg_puts_display was more complex than necessary in nvim, as in
nvim, it no longer talks directly with a terminal.

In particular we don't need to scroll the grid before emiting the last
char. The TUI already takes care of things like that, for terminals
where it matters.

test/functional/ui/screen_basic_spec.lua

@@ -795,7 +795,7 @@ local function screen_tests(linegrid)
       screen:try_resize(1, 1)
       screen:expect([[
         resize^      |
-        {2:-- INSERT -} |
+        {2:-- INSERT --}|
       ]])
 
       feed('<esc>:ls')