patch 9.0.0206: redraw flags are not named specifically

Problem:    Redraw flags are not named specifically.
Solution:   Prefix "UPD_" to the flags, for UPDate_screen().
diff --git a/src/fileio.c b/src/fileio.c
index 4287e8f..7750c52 100644
--- a/src/fileio.c
+++ b/src/fileio.c
@@ -2375,7 +2375,7 @@
 	    linecnt = 0;
 	if (newfile || read_buffer)
 	{
-	    redraw_curbuf_later(NOT_VALID);
+	    redraw_curbuf_later(UPD_NOT_VALID);
 #ifdef FEAT_DIFF
 	    // After reading the text into the buffer the diff info needs to
 	    // be updated.