drm/amdgpu: move PD/PT address calculation into backend function

This way we can better handle the differences for CPU based updates.

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