drm/amd/display: Fallback to dmcub for psr when dmcu is disabled

[Why]
We want to be able to enable/disable psr on dmcub and fallback to dmcu
when necessary.

[How]
Use dc config option to do so.

Signed-off-by: Wyatt Wood <wyatt.wood@amd.com>
Reviewed-by: Nicholas Kazlauskas <Nicholas.Kazlauskas@amd.com>
Acked-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>