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>

test/misc.c

@@ -29,6 +29,7 @@
 #include <stdint.h>
 
 #include "dix/input_priv.h"
+#include "dix/screenint_priv.h"
 #include "os/fmt.h"
 
 #include "misc.h"
@@ -66,7 +67,7 @@ dix_version_compare(void)
 
 static inline void set_screen(unsigned int idx, short x, short y, short w, short h)
 {
-    ScreenPtr pScreen = screenInfo.screens[idx];
+    ScreenPtr pScreen = dixGetScreenPtr(idx);
     pScreen->x = x;
     pScreen->y = y;
     pScreen->width = w;