drm/amd/display: Allow PSTATE chnage when no displays are enabled

[Why]
When no displays are currently enabled, display driver should not
disallow PSTATE switching.

[How]
Allow PSTATE switching if either the active configuration supports it,
or there are no active displays.

Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Aric Cyr <aric.cyr@amd.com>
Reviewed-by: Jun Lei <Jun.Lei@amd.com>
Acked-by: Anson Jacob <anson.jacob@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>