drm: add tile_group support. (v3)

A tile group is an identifier shared by a single monitor,
DisplayID topology has 8 bytes we can use for this, just
use those for now until something else comes up in the
future. We assign these to an idr and use the idr to
tell userspace what connectors are in the same tile group.

DisplayID v1.3 says the serial number must be unique for
displays from the same manufacturer.

v2:
destroy idr (dvdhrm)
add docbook (danvet)
airlied:- not sure how to make docbook add fns to tile group section.

v3: fix missing unlock.

Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Dave Airlie <airlied@redhat.com>