drm/i915: Initialize obj->pages before use by i915_gem_object_do_bit17_swizzle()

If we leave obj->pages set to NULL before attempting to deswizzle them,
then an OOPS is well deserved.

Fixes regression introduced in commit 9da3da66
Author: Chris Wilson <chris@chris-wilson.co.uk>
Date:   Fri Jun 1 15:20:22 2012 +0100

    drm/i915: Replace the array of pages with a scatterlist

Reported-and-tested-by: Krzysztof Kolasa
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>