[media] s5p-fimc: Simplify locking by removing the context data structure spinlock

Access to the memory-to-memory video node is serialized through a
mutex so now there is no point in having per device context structure
spinlock.

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>