Comment 0 for bug 1204936

Case of today:
- initializing xorg with -mir fails on ati, it exits on: [drm] failed to set drm interface version. (see dmesg http://paste.ubuntu.com/5911197/ , Xorg.log http://paste.ubuntu.com/5911199/)
lightdm then:
[+1.41s] DEBUG: Starting X server on Unity compositor
[+1.41s] DEBUG: Using VT 7
[+1.41s] DEBUG: Logging to /var/log/lightdm/x-0.log
[+1.41s] DEBUG: Writing X server authority to /var/run/lightdm/root/:0
[+1.41s] DEBUG: Launching X Server
[+1.41s] DEBUG: Launching process 4202: /usr/bin/X -core :0 -auth /var/run/lightdm/root/:0 -nolisten tcp vt7 -novtswitch
[+1.41s] DEBUG: Waiting for ready signal from X server :0
[+1.56s] DEBUG: Process 4202 terminated with signal 6
[+1.56s] DEBUG: X server stopped
[+1.56s] DEBUG: Removing X server authority /var/run/lightdm/root/:0
[+1.56s] DEBUG: Releasing VT 7
[+1.56s] DEBUG: Display server stopped
-> it tried it twice, as you can see on http://paste.ubuntu.com/5911373/.

Shouldn't lightdm tries to start xorg with -mir (it's weird I don't see that on logs, so maybe there is something elseā€¦), then fallback to traditional xorg if xorg -mir exited too early?