drm/i915: Shrink objects prior to hibernation

In an attempt to keep the hibernation image as same as possible, let's
try and discard any unwanted pages and our own page arrays.

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/20160909190218.16831-1-chris@chris-wilson.co.uk