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.