qxl: fix framebuffer unpinning

qxl_plane_cleanup_fb() unpins the just activated framebuffer
instead of the old one.  Oops.  Fix it.

Cc: Gabriel Krisman Bertazi <krisman@collabora.co.uk>
Fixes: 1277eed5
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Gabriel Krisman Bertazi <krisman@collabora.co.uk>
Link: http://patchwork.freedesktop.org/patch/msgid/20170918074145.2257-1-kraxel@redhat.com