Draft: kms/impl-device/atomic: Use non-blocking atomic commits for modesets

If there are no page flip listeners, use the OUT_FENCE_PTR CRTC property to get a sync file descriptor which signals (becomes readable) when the atomic commit finishes.

In the future, this could be extended to handle all modesets fully asynchronously, allowing them to be processed in parallel with other atomic commits by the kernel.

For now though, just wait synchronously for the sync files to signal. This should result in mostly the same behaviour as far as the rest of mutter is concerned, and should be enough to avoid hitting the RT CPU time limit in the kernel.

Issue: #3037 (closed)