Fixes the failure to start with fbdev-backend:

weston: /home/pq/git/weston/libweston/compositor.c:4733: weston_compositor_add_pending_output: Assertion `output->disable' failed.

The disable hook was completely unimplemented, and the regression was
caused by e952a01c
"libweston: move asserts to add_pending_output()".
It used to work because Weston never tried to explicitly disable the
fbdev output, but now it is hitting the assert.

Fix it by tentatively implementing a disable hook. It has not been
tested to work for explicit disabling, but it does solve the regression.

Fixes: https://bugs.freedesktop.org/show_bug.cgi?id=102208


Cc: bluescreen_avenger@verizon.net
Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
Reviewed-by: Armin Krezović <krezovic.armin@gmail.com>
Tested-by: n3rdopolis <bluescreen_avenger@verizon.net>