drm/amdgpu/gfx: add definition of mmCGTT_GS_NGG_CLK_CTRL

Needed for clockgating.

Signed-off-by: Xiaojie Yuan <xiaojie.yuan@amd.com>
Reviewed-by: Feifei Xu <Feifei.Xu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>