drm/amd/display: Set disable_dmcu flag properly per asic

[Why]
The default value for disable_dmcu is true, even for asics that require
dmcu.

[How]
Set flag properly per asic.

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>