compositor-fbdev: migrate to head-based output API

Implement the head-based output API in this backend, and stop relying on
the implicit weston_output::head.

The split between fbdev_head and fbdev_output is somewhat arbitrary.
There is no hotplug or unplug, and there is always 1:1 relationship.
Struct fbdev_screeninfo could have been split as well, but it would not
have made much difference.

I chose fbdev_output to carry the mmap details (buffer_length is now
duplicated here), and fbdev_head to carry the display parameters and
device node path. The device node identifies the head, similar to a
connector.

The backend init creates a head. The compositor uses it to create an
output. Libweston core attaches the head automatically after creating
the output. The attach hook is a suitable place to set up the video
modes on the output as they are dictated by the head, it would be too
late at enable() time.

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