weston: migrate DRM to head-based output API

Migrate the DRM frontend to use the simple head-based output
configurator, maintaining the exact same features and semantics as
before.

This is an intermediate step. It is unoptimal to create a weston_output
just to turn it off, but the libweston implementation and the DRM
backend require it for now. In the future, the DRM frontend will get its
own configurator that does not create useless weston_outputs and
supports clone mode by attaching multiple heads to the same
weston_output. Clone mode is not yet supported by libweston/DRM.

Until we remove the need to create a weston_output just to turn it
"off", that is, disable it, we will hit simple_head_enable() for heads
we have already disabled. As long as the DRM-backend conversion to the
head-based API is not complete, attempting to create an output for a
head again would lead to a crash. This problem does not exist right now,
but it will after the patch "compositor-drm: start migration to
head-based output API". Therefore, check if the head we are about to
process is already attached, and do nothing if so. DRM outputs set to
"off" are the only ones legitimately hitting this condition.

This is the last frontend migrated, wet_set_pending_output_handler() is
deleted as dead code.

v9:
- Add the workaround in simple_head_enable().

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
v6 Reviewed-by: Ian Ray <ian.ray@ge.com>
v7 Reviewed-by: Daniel Stone <daniels@collabora.com>
Acked-by: Derek Foreman <derekf@osg.samsung.com>