ui: Move check_col/check_row functions to mbyte.c

These functions were only being used by mbyte.c, so move them and add the
"static" modifier.

src/nvim/ui.c

@@ -102,26 +102,3 @@ void ui_cursor_shape(void)
   conceal_check_cursur_line();
 }
 
-/*
- * Check bounds for column number
- */
-int check_col(int col)
-{
-  if (col < 0)
-    return 0;
-  if (col >= (int)screen_Columns)
-    return (int)screen_Columns - 1;
-  return col;
-}
-
-/*
- * Check bounds for row number
- */
-int check_row(int row)
-{
-  if (row < 0)
-    return 0;
-  if (row >= (int)screen_Rows)
-    return (int)screen_Rows - 1;
-  return row;
-}