gallium: Always include xorg-server.h before other X server headers.

Various breakage otherwise, e.g. _XSERVER64 not being defined on 64 bit leading
to inconsistent definitions of X server internal structs.

src/gallium/state_trackers/xorg/xorg_dri2.c

@@ -28,6 +28,7 @@
  *
  */
 
+#include "xorg-server.h"
 #include "xf86.h"
 #include "xf86_OSproc.h"
 

src/gallium/state_trackers/xorg/xorg_exa.c

@@ -28,6 +28,7 @@
  *
  */
 
+#include "xorg-server.h"
 #include "xf86.h"
 #include "xorg_tracker.h"