compositor-drm: simplify drm_output_find_by_crtc()

As CRTC is allocated on output enable and deallocated on output disable,
there cannot be any matches in find-by-crtc from the
pending_output_list.

Remove the loop over pending_output_list as never finding anything by
definition.

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>