drm/amdgpu: update one PDE at a time v2

Horrible inefficient, but avoids problems when the root PD size becomes
to big.

v2: remove incr as well.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Chunming Zhou <davdi1.zhou@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>