ui: Only call ui_flush when the busy state changes

Also add back the `ui_flush` call to `get_keystroke`. Its necessary to display
prompt messages correctly.

src/nvim/misc1.c

@@ -2364,7 +2364,7 @@ int get_keystroke(void)
 
   mapped_ctrl_c = FALSE;        /* mappings are not used here */
   for (;; ) {
-
+    ui_flush();
     /* Leave some room for check_termcode() to insert a key code into (max
      * 5 chars plus NUL).  And fix_input_buffer() can triple the number of
      * bytes. */

src/nvim/ui.c

@@ -155,16 +155,16 @@ void ui_busy_start(void)
 {
   if (!(busy++)) {
     UI_CALL(busy_start);
+    ui_flush();
   }
-  ui_flush();
 }
 
 void ui_busy_stop(void)
 {
   if (!(--busy)) {
     UI_CALL(busy_stop);
+    ui_flush();
   }
-  ui_flush();
 }