amdgpu/dc: convert dc_transfer to use a kref.

Rolling your own atomic ref counts is frowned upon.

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>