drm/cirrus: Fix NULL pointer dereference when registering the fbdev

cirrus_modeset_init() is initializing/registering the emulated fbdev
and, since commit c61b93fe51b1 ("drm/atomic: Fix remaining places where
!funcs->best_encoder is valid"), DRM internals can access/test some of
the fields in mode_config->funcs as part of the fbdev registration
process.
Make sure dev->mode_config.funcs is properly set to avoid dereferencing
a NULL pointer.

Reported-by: Mike Marshall <hubcap@omnibond.com>
Reported-by: Eric W. Biederman <ebiederm@xmission.com>
Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Fixes: c61b93fe51b1 ("drm/atomic: Fix remaining places where !funcs->best_encoder is valid")
Signed-off-by: Dave Airlie <airlied@redhat.com>