treewide: use helper dixGetScreenPtr() for retrieving ScreenPtr's

Instead of directly accessing the global screenInfo.screens[] array,
let everybody go through a little inline helper. This one also checks
for array bounds - if the screen doesn't exist, return NULL.

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

randr/rrxinerama.c

@@ -74,6 +74,7 @@
 
 #include "dix/dix_priv.h"
 #include "dix/request_priv.h"
+#include "dix/screenint_priv.h"
 #include "include/extinit.h"
 #include "randr/randrstr_priv.h"
 
@@ -313,7 +314,7 @@ RRXineramaExtensionInit(void)
      * with their own output geometry.  So if there's more than one protocol
      * screen, just don't even try.
      */
-    if (screenInfo.numScreens > 1)
+    if (dixGetScreenPtr(1))
         return;
 
     (void) AddExtension(PANORAMIX_PROTOCOL_NAME, 0, 0,