drm/amdgpu: disable DC on KB/ML for now

Until I've had time to test it better.

bug: https://bugs.freedesktop.org/show_bug.cgi?id=102372



Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>