drm/amdgpu: Correct the ndw of bo update mapping.

For buffer object that has shadow buffer, need twice commands.

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