drm/amd/display: Use config flag to disable dmcu obj creation

[Why]
When dmcub is the default we no longer wish to create the psr and dmcu
objects. Currently a dc debug flag is used to implement this, but these
flags aren't populated until after dcn21_resource_construct is called.
This means the dmcub objects will never be created. Therefore we must
use a dc config flag, which is populated before dc resource construct.

[How]
Add a dc config flag.

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>