drm/i915: Protect against deferred fbdev setup

We could probably hit this already with our current async fbdev init,
but it's much easier to hit this with the new deferred fbdev setup
that I'm working on polishing.

Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Reported-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170706130023.28417-2-daniel.vetter@ffwll.ch