drm/amd/display: dc cleanup for tests

[WHY&HOW]
Code cleanup found in internal tests

Reviewed-by: Dillon Varone <dillon.varone@amd.com>
Acked-by: Stylon Wang <stylon.wang@amd.com>
Signed-off-by: Sridevi Arvindekar <Sridevi.Arvindekar@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>