Backport "modesetting: Disable atomic support by default" to server-1.20-branch

As mentioned in the original commit message:

Please backport to 1.20, as we don't want to enable it for everyone there. It breaks for existing users.

Unfortunately, this is fix was never backported to the stable branch.

Without this, the kernel disregards the Xserver requests as "broken atomic modeset userspace detected, disabling atomic".