drm/amdgpu: correct ip for mmHDP_READ_CACHE_INVALIDATE register access

Use the proper IP index for HDP registers.

Signed-off-by: Le Ma <le.ma@amd.com>
Reviewed-by: Feifei Xu <Feifei.Xu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>