[media] exynos4-is: Media graph/video device locking rework

Remove driver private video node reference counters and use entity->use_count
instead. This makes the video pipelines power handling more similar to the
method used in omap3isp driver.
Now the graph mutex is taken always after the video mutex, as it is not
possible to ensure apposite order at the all modules.

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>