drm/i915: fixup pd vs pt confusion in gen6 ppgtt code

The index variable points at a page table, not a page directory or a
pde. Ben Widawsky fix this up correctly in his ppgtt cleanup, but I've
botched the job and copy&pasted the old confusion from the original
gen6 ppgtt code in

commit def886c3768d24c4e0aa56ff98b5a468c2b5c9bf
Author: Daniel Vetter <daniel.vetter@ffwll.ch>
Date:   Thu Jan 24 14:44:56 2013 -0800

    drm/i915: vfuncs for ppgtt

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