drm/doc: document DRM_IOCTL_SYNCOBJ_EVENTFD

drm/doc: document DRM_IOCTL_SYNCOBJ_EVENTFD

struct drm_syncobj_eventfd was documented, but
DRM_IOCTL_SYNCOBJ_EVENTFD was not. This prevents references to this
define from being properly linkified in docs.

Signed-off-by: Simon Ser <contact@emersion.fr>
Reviewed-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.com>
Cc: Simona Vetter <simona@ffwll.ch>
Cc: Daniel Stone <daniel@fooishbar.org>
Cc: Michel Dänzer <michel.daenzer@mailbox.org>
Link: https://patch.msgid.link/20260327171812.128290-1-contact@emersion.fr