drm/i915: Drain freed objects for mmap space exhaustion

As we now use a deferred free queue for objects, simply retiring the
active objects is not enough to immediately free them and recover their
mmap space - we must now also drain the freed object list.

Fixes: fbbd37b3 ("drm/i915: Move object release to a freelist + worker"
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: <drm-intel-fixes@lists.freedesktop.org>
Link: http://patchwork.freedesktop.org/patch/msgid/20170106152240.5793-3-chris@chris-wilson.co.uk


Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>