backends/native: Add xlnx to the atomic deny list

Although its atomic KMS support seems to work at first, mode sets to
anything other than the Xilinx preferred max resolution of 2048x1280
would result in a hang. The xlnx kernel driver is given:
`DRM_MODE_ATOMIC_ALLOW_MODESET | DRM_MODE_PAGE_FLIP_EVENT`
and it does complete the mode set without error, but page flip events
never arrive and so you're frozen on the first frame.

Revert to legacy KMS which has no such problem with non-default modes.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/2596>

src/backends/native/meta-kms-impl-device-atomic.c

@@ -1216,6 +1216,7 @@ is_atomic_allowed (const char *driver_name)
     "vboxvideo",
     "nvidia-drm",
     "virtio_gpu",
+    "xlnx",
     NULL,
   };