Was missing before and would have resulted in a write to
a non-existant register. Normally APUs don't use HDP, but
other asics could use this code and APUs do use the HDP
when used in passthrough.
Reviewed-by: Lijo Lazar <lijo.lazar@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>