amdgpu/dc: hide some unused aux/i2c payload apis.

I've no idea if these are used on other platforms, but they
aren't used outside this file here, so make them static.

Drops 300 bytes.

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>