drm/amd/display: Prevent Diags from entering S2

[why]
Lowering clocks when entering S2 Idle state causes DMUB to hang with
Diags.

[how]
Do not enter S2 optimization with Diags on dcn301 to prevent DMUB hang.

Acked-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Mikita Lipski <mikita.lipski@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>