drm/amdgpu: remove amdgpu_bo_list_entry.robj (v2)

We can get that just by casting tv.bo.

v2: squash in kfd fix (Alex)

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