compositor-drm: simplify drm_output_find_special_plane()

As these planes are allocated on output enable and freed on output
disable, there cannot be a match in the pending_output_list.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
Reviewed-by: Daniel Stone <daniels@collabora.com>
Acked-by: Derek Foreman <derekf@osg.samsung.com>