compositor-drm: set output make/model/serial once

This fixes a regression where monitor make and model would always be
advertised as "unknown" to Wayland clients. The EDID strings were parsed
at create_output_for_connector() time, but the fallback "unknown" values
were set in weston_drm_output_api::set_mode vfunc later. This made the
correct monitor info be shown in the log, but not sent to clients.

The purpose of the "unknown" assignments is to give fallback values in
case EDID is not providing them.

Fix all that by moving all setting of the make, model and serial into
create_output_for_connector(). These values cannot change afterwards
anyway. While at it, document find_and_parse_output_edid().

The ugly casts in create_output_for_connector() are required to silence
compositor warnings from ignoring const attribute. This is temporary,
and a future refactoring will get rid of the casts.

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