media: vimc: fix start stream when link is disabled

If link is disabled, media_entity_remote_pad returns NULL, causing a
NULL pointer deference.
Ignore links that are not enabled instead.

Signed-off-by: Helen Koike <helen.koike@collabora.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>