drm/radeon: program auxch directly (v2)

The atombios tables have an unfortunate restriction on only
being able to write 12 bytes, MST really wants 16-bytes here,
and since the hw can do it, we should just write directly to it.

This uses a module option to allow for it now, and maybe
we should provide the old code as a fallback for a while.

v2: (agd5f)
- move registers to a proper register header
- only enable on DCE5+
- enable by default on DCE5+
- Switch pad to aux mode before using it
- reformat instance handling to better match the
  rest of the driver

Signed-off-by: Dave Airlie <airlied@redhat.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>