drm/amdgpu: nuke amdgpu_vm_bo_split_mapping v2

Merge the functionality mostly into amdgpu_vm_bo_update_mapping.

This way we can even handle small contiguous system pages without
to much extra CPU overhead.

v2: fix typo, keep the cursor as it is for now

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Madhav Chauhan <madhav.chauhan@amd.com> (v1)
Reviewed-by: Felix Kuehling <Felix.Kuehling@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>