drm/syncobj: Fix syncobj leak in drm_syncobj_eventfd_ioctl

drm/syncobj: Fix syncobj leak in drm_syncobj_eventfd_ioctl

A syncobj reference is taken in drm_syncobj_find, but not released if
eventfd_ctx_fdget or kzalloc fails. Put the reference in these error
paths.

Reported-by: Xingyu Jin <xingyuj@google.com>
Fixes: c7a472297169 ("drm/syncobj: add IOCTL to register an eventfd")
Signed-off-by: T.J. Mercier <tjmercier@google.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@igalia.com>
Reviewed-by. Christian König <christian.koenig@amd.com>
CC: stable@vger.kernel.org # 6.6+
Link: https://patchwork.freedesktop.org/patch/msgid/20240909205400.3498337-1-tjmercier@google.com
Signed-off-by: Christian König <christian.koenig@amd.com>