glx/dri2: Disable AIGLX if indirect GLX is disabled

There is no point in setting up the acceleration for indirect GLX if
indirect GLX is itself disabled.

Reviewed-by: Adam Jackson <ajax@redhat.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>

glx/glxdri2.c

@@ -936,6 +936,9 @@ __glXDRIscreenProbe(ScreenPtr pScreen)
     size_t buffer_size;
     ScrnInfoPtr pScrn = xf86ScreenToScrn(pScreen);
 
+    if (!enableIndirectGLX)
+	    return NULL;
+
     screen = calloc(1, sizeof *screen);
     if (screen == NULL)
         return NULL;