drm/qxl: properly free qxl releases

Reorganize qxl_device_fini() a bit.
Add missing unpin() calls.

Count releases.  Add wait queue for releases.  That way
qxl_device_fini() can easily wait until everything is
ready for proper shutdown.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: http://patchwork.freedesktop.org/patch/msgid/20210204145712.1531203-9-kraxel@redhat.com