drm/amdgpu: fix wait queue handling in the scheduler

Freeing up a queue after signalling it isn't race free.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Jammy Zhou <Jammy.Zhou@amd.com>
Reviewed-by: Chunming Zhou <david1.zhou@amd.com>
2 files changed