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>