drm/i915: Refactor unsettting obj->mm.pages

As i915_gem_object_phys_attach() wants to play dirty and mess around
with obj->mm.pages itself (replacing the shmemfs with a DMA allocation),
refactor the gubbins so into i915_gem_object_unset_pages() that we don't
have to duplicate all the secrets.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180611075532.26534-1-chris@chris-wilson.co.uk
Link: https://patchwork.freedesktop.org/patch/msgid/152871104647.1718.8796913290418060204@jlahtine-desk.ger.corp.intel.com