xf86: select a fake output for headless servers

Following commit 37d956e3ac
Author: Keith Packard <keithp@keithp.com>
Date:   Mon Sep 10 11:14:20 2012 +1000

    xf86: fix compat output selection for no output GPUs

headless servers can no longer startup as we no longer select a compat
output for the fake framebuffer.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=56343
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Dave Airlie <airlied@redhat.com>
Signed-off-by: Keith Packard <keithp@keithp.com>

hw/xfree86/modes/xf86Crtc.c

@@ -1847,6 +1847,10 @@ SetCompatOutput(xf86CrtcConfigPtr config)
         output = config->output[config->compat_output];
     }
 
+    /* All outputs are disconnected, select one to fake */
+    if (!output && config->num_output)
+        output = config->output[0];
+
     return output;
 }