[media] exynos4-is: Add locking at fimc(-lite) subdev unregistered handler

Protect the fimc/fimc-lite video nodes unregistration with their video
lock. This prevents a kernel crash when e.g. udev opens a video node
right after the driver registers it and then the driver tries to
unregister it and defers its probing. Using video_is_unregistered()
together with the video mutex allows safe unregistration of the video
nodes at any time.

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>