drm/i915: Do not allow buffers at offset 0

This is primarily a band aid for an unexplainable error in
gem_reloc_vs_gpu/forked-faulting-reloc-thrashing. Essentially as soon as
a relocated buffer (which had a non-zero presumed offset) moved to
offset 0, something goes bad. Since I have been unable to solve this,
and potentially this is a good thing to do anyway, since many things can
accidentally write to offset 0, why not?

Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>