drm/syncobj: add IOCTL to register an eventfd

Introduce a new DRM_IOCTL_SYNCOBJ_EVENTFD IOCTL which signals an
eventfd from a syncobj.

This is useful for Wayland compositors to handle wait-before-submit.
Wayland clients can send a timeline point to the compositor
before the point has materialized yet, then compositors can wait
for the point to materialize via this new IOCTL.

The existing DRM_IOCTL_SYNCOBJ_TIMELINE_WAIT IOCTL is not suitable
because it blocks. Compositors want to integrate the wait with
their poll(2)-based event loop.

Requirements for new uAPI:

- User-space patch: https://gitlab.freedesktop.org/wlroots/wlroots/-/merge_requests/4262
- IGT: https://lists.freedesktop.org/archives/igt-dev/2023-July/057893.html

v2:
- Wait for fence when flags is zero
- Improve documentation (Pekka)
- Rename IOCTL (Christian)
- Fix typo in drm_syncobj_add_eventfd() (Christian)

v3:
- Link user-space + IGT patches
- Add reference from overview docs

v4: fix IOCTL number conflict with GETFB2 (Nicholas Choi, Vitaly Prosyak)

Signed-off-by: Simon Ser <[email protected]>
Reviewed-by: Christian König <[email protected]>
Acked-by: Pekka Paalanen <[email protected]>
Cc: Jason Ekstrand <[email protected]>
Cc: Daniel Vetter <[email protected]>
Cc: Bas Nieuwenhuizen <[email protected]>
Cc: Daniel Stone <[email protected]>
Cc: James Jones <[email protected]>
Cc: Austin Shafer <[email protected]>
Cc: Vitaly Prosyak <[email protected]>
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]

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