[media] call media_device_init() before registering the V4L2 device

Currently, v4l2_device_register() doesn't use the media_device
struct. So, calling media_device_init() could be called either
before or after v4l2_device_register().

Yet, it is a good practice to initialize everything before calling
the register functions. Also, the other drivers call
media_device_init() before registering the V4L2 device.

So, move the call for media_device_init() to happen earlier on
exynos4-is and s3c-camif.

This is just a cleanup patch.

Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>