drm/amdgpu: add new asic callbacks for HDP flush/invalidation

Needed to properly flush the HDP cache with the CPU from rather
than the GPU.

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