drm/i915: Skip shrinking already freed pages

Previously, we wanted to shrink the pages of freed objects before they
were finally RCU collected. However, by removing the struct_mutex
serialisation around the active reference, we need to acquire an extra
reference around the wait. Unfortunately this means that we have to skip
objects that are waiting RCU collection.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=110937


Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190618074153.16055-2-chris@chris-wilson.co.uk