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>