drm/amdgpu: Enable MGCG on SDMAv4.4.2

Enable clock gating on SDMAv4.4.2 versions. Leave memory light sleep to
default.

Signed-off-by: Lijo Lazar <lijo.lazar@amd.com>
Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>