drm/msm/gem: Unpin buffers earlier

We've already attached the fences, so obj->resv (which shrinker checks)
tells us whether they are still active.  So we can unpin sooner, before
we drop the queue lock.

This also avoids the need to grab the obj lock in the retire path,
avoiding potential for lock contention between submit and retire.

Signed-off-by: Rob Clark <robdclark@chromium.org>
Patchwork: https://patchwork.freedesktop.org/patch/496132/
Link: https://lore.kernel.org/r/20220802155152.1727594-12-robdclark@gmail.com