drm/amdgpu/gfx9: fix ngg enablement to clear gds reserved memory (v2)

v2: fix register access

Signed-off-by: Junwei Zhang <Jerry.Zhang@amd.com>
Reviewed-by: Ken Wang <ken.wang@amd.com>
Acked-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>