drm/i915: fixup i915_gem_object_get_page inline helper

drm/i915: fixup i915_gem_object_get_page inline helper

Note that just because we have n == MAX elements left, does not imply
that there are only MAX elements left in the scatterlist and so we may
not be on the last chain, and the nth element may in fact be a chain ptr.

This is exercised by the improved hangman tests and the gem_exec_big
test in i-g-t.

This regression has been introduced in

commit eca3e5d5e804d03fb5c1bbf8a9ad347f196ff953
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

v2: KISS, replace the direct lookup with a for_each_sg() [danvet]
v3: Try to be clever again.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>