drm/syncobj: remove drm_syncobj_cb and cleanup

This completes "drm/syncobj: Drop add/remove_callback from driver
interface" and cleans up the implementation a bit.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Chunming Zhou <david1.zhou@amd.com>
Link: https://patchwork.freedesktop.org/patch/266255/