drm/amdgpu: add HDP asic callbacks for SOC15 (v2)

Needed to flush and invalidate the HDP block using the CPU.

v2: use preferred register on soc15.

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> (v1)