drm/qxl: simplify qxl_fence_wait

Now that we have the new release_event wait queue we can just
use that in qxl_fence_wait() and simplify the code a lot.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: http://patchwork.freedesktop.org/patch/msgid/20210204145712.1531203-10-kraxel@redhat.com