drm/amdgpu: Enable NPS4 CPX mode

CPX compute mode is valid mode for NPS4 memory partition mode.

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