drm/simpledrm: Fix power domain device link validity check

We need to check if a link is non-NULL before trying to delete it.

Fixes: 61df9ca2 ("drm/simpledrm: Add support for multiple "power-domains"")
Signed-off-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Thomas Zimmermann <tzimmermann@suse.de>
Cc: Janne Grunau <j@jannau.net>
Cc: Eric Curtin <ecurtin@redhat.com>
Cc: Neal Gompa <neal@gompa.dev>
Cc: Sven Peter <sven@svenpeter.dev>
Cc: Javier Martinez Canillas <javierm@redhat.com>
Cc: dri-devel@lists.freedesktop.org
Link: https://patchwork.freedesktop.org/patch/msgid/20231011143230.1107731-1-thierry.reding@gmail.com