Just add the interfaces to attach/detach output slaves, and
a linked list to keep track of them. Hook up the randr providers
list to include these slaves.
v1.1: add another assert to the add path.
Reviewed-by: Keith Packard <keithp@keithp.com>
Reviewed-by: Adam Jackson <ajax@redhat.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>