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.

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

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>