drm/amdgpu: remove DC special casing for KB/ML

It seems to be working now.

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


Reviewed-by: Mike Lothian <mike@fireburn.co.uk>
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>