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>

drivers/gpu/drm/i915/i915_gem.c

@@ -1825,10 +1825,11 @@ i915_gem_object_get_pages_gtt(struct drm_i915_gem_object *obj)
 		sg_set_page(sg, page, PAGE_SIZE, 0);
 	}
 
+	obj->pages = st;
+
 	if (i915_gem_object_needs_bit17_swizzle(obj))
 		i915_gem_object_do_bit_17_swizzle(obj);
 
-	obj->pages = st;
 	return 0;
 
 err_pages: