amdgpu/dc: use kref for dc_state.

I'm not a huge fan of those copying around refcounts bits, might
want to consider alternates, but this should work for now.

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