drm/amdgpu: Fix dangling kfd_bo pointer for shared BOs

If a kfd_bo was shared (e.g. a dmabuf export), the original kfd_bo may be
freed when the amdgpu_bo still lives on. Free the kfd_bo struct in the
release_notify callback then the amdgpu_bo is freed.

Signed-off-by: Felix Kuehling <Felix.Kuehling@amd.com>
Reviewed-By: Ramesh Errabolu <Ramesh.Errabolu@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>