DRI: Log something if we don't support legacy DRI

If we are called via the legacy DRI interface, and we don't support
legacy DRI (InitScreen is NULL), print a debug message, so it is easy
to see why the driver fails to initialize.

See https://bugs.freedesktop.org/show_bug.cgi?id=40437

src/mesa/drivers/dri/common/dri_util.c

@@ -816,8 +816,10 @@ driCreateNewScreen(int scrn,
 
     (void) loaderPrivate;
 
-    if (driDriverAPI.InitScreen == NULL)
+    if (driDriverAPI.InitScreen == NULL) {
+	__driUtilMessage("driver does not support DRI1");
 	return NULL;
+    }
 
     psp = calloc(1, sizeof *psp);
     if (!psp)