drm/amdgpu: add hdp version 6 functions

Unify hdp related function into hdp structure for hdp version 6.
V2: Remove hdp invalidate function as hdp v6 doesn't have read cache.

Signed-off-by: Likun Gao <Likun.Gao@amd.com>
Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>