kms/impl-device/atomic: Set IN_FENCE_FD for direct scanout

Since meta_kms_impl_device_get_sync_file always returns the same file descriptor referencing the same sync_file, this means the atomic ioctl doesn't need to wait for any fences to signal. This is fine because we already waited for the buffer to become idle before applying the Wayland surface state.

Fixes the atomic commit ioctl spuriously synchronizing to the screen cast paint (at least with the amdgpu driver), which could result in the page flip missing its target scanout cycle.

/cc @YaLTeR