treewide: use inline helper for accessing first screen

Instead of everybody directly accessing the (internal) screenInfo struct,
let those consumers only interested in first screen use a little helper.

Also caching the value if it's needed several times.

Signed-off-by: Enrico Weigelt, metux IT consult <info@metux.net>

xfixes/cursor.c

@@ -49,6 +49,7 @@
 #include "dix/input_priv.h"
 #include "dix/rpcbuf_priv.h"
 #include "dix/screen_hooks_priv.h"
+#include "dix/screenint_priv.h"
 
 #include "xfixesint.h"
 #include "scrnintstr.h"
@@ -618,7 +619,7 @@ ReplaceCursor(CursorPtr pCursor, TestCursorFunc testCursor, void *closure)
         }
     }
     /* this "knows" that WindowHasNewCursor doesn't depend on its argument */
-    WindowHasNewCursor(screenInfo.screens[0]->root);
+    WindowHasNewCursor(dixGetFirstScreenPtr()->root);
 }
 
 static Bool