Allow to enable atomic in modesetting DDX
The linux kernel blacklisted atomic for the modesetting DDX to deal with problems with old Xorg versions which didn't have it disabled by default. Ever since, it wasn't possible to enable atomic in modesetting at all anymore. This change doesn't enable atomic by default, but it allows a user to enabled it again. Signed-off-by: Daniel Abrecht <public@danielabrecht.ch>