drm/amdgpu: nuke amdgpu_bo_list_free

The RCU grace period is harmless and avoiding it is not worth the effort
of doubling the implementation.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Chunming Zhou <david1.zhou@amd.com>
Reviewed-by: Huang Rui <ray.huang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>