[media] s5p-fimc: Avoid crash with null platform_data

In commit "s5p-fimc: Handle sub-device interdependencies using deferred.."
there was a check added for pdata->num_clients without first checking
pdata against NULL. This causes a crash when platform_data is not set,
which is a valid use case. Fix this regression by skipping the MIPI-CSIS
subdev registration also when pdata is null.

Reported-by: HeungJun Kim <riverful.kim@samsung.com>
Signed-off-by: Sylwester Nawrocki <s.nawrocki@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>