drm/dp: Add DPCD OUI register defines

DisplayPort has an escape hatch by which sources and sinks can identify
each other.  We would prefer not to notice this, but I suspect we're
going to need to.

Signed-off-by: Adam Jackson <ajax@redhat.com>
Reviewed-by: Jerome Glisse <jglisse@redhat.com>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>