compositor-fbdev: make copy of the device string

compositor-fbdev: make copy of the device string

Ensuring that the pointer to the device path stays valid gets harder and
harder with migrating to the libweston-style config handling. Therefore,
make a copy of the string, private to struct fbdev_output.

Now the pointer passed in to fbdev_output_create() could be freed right
after the call returns.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
Reviewed-by: Quentin Glidic <sardemff7+git@sardemff7.net>
Reviewed-by: Benoit Gschwind <gschwind@gnu-log.net>