drm/amd/display: enable idle optimizations for beige goby

[Why&How]
MALL requires idle optimizations to be enabled. This enables MALL
feature on dcn303

Signed-off-by: Aurabindo Pillai <aurabindo.pillai@amd.com>
Reviewed-by: Tao Zhou <tao.zhou1@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>