drm/fb_helper: prevent some troubles waiting to happen

[Ilija Hadzic]

The following two patches address potential problems that I 
called "troubles waiting to happen" in this note
http://lists.freedesktop.org/archives/dri-devel/2011-October/015412.html

I didn't hear anyone take on my question, so I figured I would just send
the patches. I tested the patches on a variety of AMD cards that I have.
I don't have other GPUs handy.