drm/i915: s/any_enabled/!fallback/ in fbdev_initial_config

It started as a simple check whether anything is lit up, but now is't
used to driver the general fallback logic to the default output
configuration selector in the helper library. So rename it for more
clarity.

Cc: Jesse Barnes <jbarnes@virtuousgeek.org>
Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>