drm/amdgpu: enable clock gating for HDP 6.0

Enable HDP 6.0 clock gating.

Signed-off-by: Evan Quan <evan.quan@amd.com>
Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>