As we have dropped the final reference to the object, we do not need to
wait until after the rcu grace period to drop its pages. We still require
struct_mutex to completely unbind the object to release the pages, so we
still need a free-worker to manage that from process context. By
scheduling the release of pages before waiting for the rcu should mean
that we are not trapping those pages from beyond the reach of the
shrinker.

v2: Pass along the request to skip if the vma is busy to the underlying
unbind routine, to avoid checking the reservation underneath the
i915->mm.obj_lock which may be used from inside irq context.

v3: Flip the bit for unbinding while active, for later convenience.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=111035
Fixes: a93615f9 ("drm/i915: Throw away the active object retirement complexity")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Matthew Auld <matthew.auld@intel.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190703091726.11690-6-chris@chris-wilson.co.uk