drm/amdgpu: Support page table update via CPU

v2: Fix logical mistake. If CPU update failed amdgpu_vm_bo_update_mapping()
would not return and instead fall through to SDMA update. Minor change due to
amdgpu_vm_bo_wait() prototype change

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