drm: fix deadlock of syncobj v6

v2:
add a mutex between sync_cb execution and free.
v3:
clearly separating the roles for pt_lock and cb_mutex (Chris)
v4:
the cb_mutex should be taken outside of the pt_lock around
this if() block. (Chris)
v5:
fix a corner case
v6:
tidy drm_syncobj_fence_get_or_add_callback up. (Chris)

Tested by syncobj_basic and syncobj_wait of igt.

Signed-off-by: Chunming Zhou <david1.zhou@amd.com>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Christian König <christian.koenig@amd.com>
Cc: intel-gfx@lists.freedesktop.org
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Christian König <christian.koenig@amd.com>
Link: https://patchwork.kernel.org/patch/10652893/