drm/amdgpu: create an empty bo_list if no handle is provided

Instead of having extra handling just create an empty bo_list when no
handle is provided.

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>