drm/i915: Always set the vma->pages

Previously, we would only set the vma->pages pointer for GGTT entries.
However, if we always set it, we can use it to prettify some code that
may want to access the backing store associated with the VMA (as
assigned to the VMA).

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1471254551-25805-8-git-send-email-chris@chris-wilson.co.uk