drm/qxl: more fence wait rework

Move qxl_io_notify_oom() call into wait condition.
That way the driver will call it again if one call
wasn't enough.

Also allows to remove the extra dma_fence_is_signaled()
check and the goto.

Fixes: 5a838e5d

 ("drm/qxl: simplify qxl_fence_wait")
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: http://patchwork.freedesktop.org/patch/msgid/20210217123213.2199186-3-kraxel@redhat.com