drm/i915: fix NULL deref in the load detect code

Looks like I've missed one of the potential NULL deref bugs in Jesse's
fbdev->fb embedded struct to pointer conversions. Fix it up.

This regression has been introduced in

commit 8bcd4553
Author: Jesse Barnes <jbarnes@virtuousgeek.org>
Date:   Fri Feb 7 12:10:38 2014 -0800

    drm/i915: alloc intel_fb in the intel_fbdev struct

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