drm/amdgpu: clarify DC checks

There are several places where we don't want to check
if a particular asic could support DC, but rather, if
DC is enabled.  Set a flag if DC is enabled and check
for that rather than if a device supports DC or not.

Acked-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c

@@ -4592,6 +4592,7 @@ static int dm_early_init(void *handle)
 		adev_to_drm(adev)->dev,
 		&dev_attr_s3_debug);
 #endif
+	adev->dc_enabled = true;
 
 	return 0;
 }