drm: Fix use-after-free in the shadow-attache exit code

drm: Fix use-after-free in the shadow-attache exit code

This regression has been introduced in

commit b3f2333de8e81b089262b26d52272911523e605f
Author: Daniel Vetter <[email protected]>
Date:   Wed Dec 11 11:34:31 2013 +0100

    drm: restrict the device list for shadow attached drivers

Reported-by: Dave Jones <[email protected]>
Cc: Dave Jones <[email protected]>
Cc: Dave Airlie <[email protected]>
Cc: David Herrmann <[email protected]>
Signed-off-by: Daniel Vetter <[email protected]>
Reviewed-by: David Herrmann <[email protected]>