drm/i915: Check the framebuffer offset

The current code can't deal with framebuffers with an offset. Return an
error when trying to create such a framebuffer until the rest of the
code is fixed to handle them.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>