drm/qxl: add lock asserts to qxl_bo_vmap_locked + qxl_bo_vunmap_locked

Try avoid re-introducing locking bugs.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: http://patchwork.freedesktop.org/patch/msgid/20210217123213.2199186-12-kraxel@redhat.com