drm/amdgpu: grab the id mgr lock while accessing passid_mapping

[ Upstream commit 6817bf28 ]

Need to make sure that we actually dropping the right fence.
Could be done with RCU as well, but to complicated for a fix.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Chunming Zhou <david1.zhou@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>