drm/amd/display: Enable DCN low mem power by default

Enable DCN low mem power by default.

Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Reviewed-by: Charlene Liu <charlene.liu@amd.com>
Acked-by: Rodrigo Siqueira <rodrigo.siqueira@amd.com>
Signed-off-by: Muhammad Ahmed <ahmed.ahmed@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>