drm: introduce CLOSEFB IOCTL

This new IOCTL allows callers to close a framebuffer without
disabling planes or CRTCs. This takes inspiration from Rob Clark's
unref_fb IOCTL [1] and DRM_MODE_FB_PERSIST [2].

User-space patch for wlroots available at [3]. IGT test available
at [4].

v2: add an extra pad field just in case we want to extend this IOCTL
in the future (Pekka, Sima).

[1]: https://lore.kernel.org/dri-devel/20170509153654.23464-1-robdclark@gmail.com/
[2]: https://lore.kernel.org/dri-devel/20211006151921.312714-1-contact@emersion.fr/
[3]: https://gitlab.freedesktop.org/wlroots/wlroots/-/merge_requests/4394
[4]: https://lists.freedesktop.org/archives/igt-dev/2023-October/063294.html

Signed-off-by: Simon Ser <contact@emersion.fr>
Reviewed-by: Daniel Stone <daniels@collabora.com>
Acked-by: Pekka Paalanen <pekka.paalanen@collabora.com>
Cc: Hans de Goede <hdegoede@redhat.com>
Cc: Dennis Filder <d.filder@web.de>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: Rob Clark <robdclark@gmail.com>
Cc: Sean Paul <seanpaul@chromium.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20231020101926.145327-2-contact@emersion.fr