drm/amdgpu: revert "use the new cursor in the VM code"

We are seeing VM page faults with this. Revert the change until the bugs
are fixed.

This reverts commit 94ae8dc5.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Guchun Chen <guchun.chen@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>