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/mbyte.c

@@ -103,7 +103,6 @@
 #include "nvim/screen.h"
 #include "nvim/spell.h"
 #include "nvim/strings.h"
-#include "nvim/ui.h"
 #include "nvim/os/os.h"
 #include "nvim/arabic.h"
 
@@ -3972,3 +3971,23 @@ char_u * string_convert_ext(vimconv_T *vcp, char_u *ptr, int *lenp,
 
   return retval;
 }
+
+// Check bounds for column number
+static 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
+static int check_row(int row)
+{
+  if (row < 0)
+    return 0;
+  if (row >= (int)screen_Rows)
+    return (int)screen_Rows - 1;
+  return row;
+}