[media] s5p-fimc: Allow probe() to succeed with null platform data

The "s5p-fimc-md" platform device platform_data is used to pass
attached camera sensor data. Not allowing device probe() to succeed
when it's null prevents using FIMC as a mem-to-mem device only.
Fix this by removing the platform_data check against null and
registering sensors only if platform_data is specified.
Also add logging of the information which /dev/video is assigned
to which device during probe().

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>