drivers/gpu/drm/drm_syncobj.c:181:6: warning: no previous prototype for ‘drm_syncobj_add_callback’ [-Wmissing-prototypes]
drivers/gpu/drm/drm_syncobj.c:190:6: warning: no previous prototype for ‘drm_syncobj_remove_callback’ [-Wmissing-prototypes]

Fixing that leads to

drivers/gpu/drm/drm_syncobj.c:181:13: warning: ‘drm_syncobj_add_callback’ defined but not used [-Wunused-function]

so remove the unused drm_syncobj_add_callback() entirely.
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20181031120710.21582-1-chris@chris-wilson.co.ukReviewed-by: Chunming Zhou <david1.zhou@amd.com>