libweston: set backend pointer earlier

Change all backends to set the core backend pointer early.

This is necessary for libweston core to be able to access the backend
vfuncs before the backend init function returns. Particularly,
weston_output_init() will be needing to inspect the backend vfuncs to
see if the backend has been converted to a new API. Backends that create
outputs as part of their init would fail without setting the pointer
earlier.

For consistency, all backends are modified instead of just those that
could hit an issue.

Libweston core will take care of resetting the backend pointer to NULL
in case of error since "libweston: ensure backend is not loaded twice".

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