drm/amdgpu: fix initializing the VM BO shadow

We need to clear the shadows as well.

Signed-off-by: Christian König <christian.koenig@amd.com>
Acked-by: Edward O'Callaghan <funfunctor@folklore1984.net>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>