[media] s5p-mfc: optimized code related to working contextes

All code setting/clearing working context bits has been moved
to separate functions. set_bit/clear_bit have been replaced by
non-atomic variants - variable is already guarded by spin_lock.

Signed-off-by: Andrzej Hajda <a.hajda@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Sylwester Nawrocki <s.nawrocki@samsung.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>