drm/i915: Lift has-pinned-pages assert to caller of ____i915_gem_object_get_pages

i915_gem_object_attach_phys() is trying to swap out its shmemfs pages
for a new set of physically contiguous pages, but unfortunately triggers
an assert inside get-pages.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=102561
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20170906135220.13508-1-chris@chris-wilson.co.uk
Reviewed-by: Matthew Auld <matthew.auld@intel.com>