drm/amdgpu: amdgpu_device_recover_vram got NULL of shadow->parent

amdgpu_bo_destroy had a bug by calling amdgpu_bo_unref outside mutex_lock.
If amdgpu_device_recover_vram executed between amdgpu_bo_unref and list_del_init,
it would get NULL of shadow->parent, then caused Call Trace and GPU reset failed.

Signed-off-by: Wentao Lou <Wentao.Lou@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>