[media] s5p-mfc: use spinlock to protect MFC context

MFC driver uses dev->irqlock spinlock to protect queues only, but many context
fields require protection also - they can be accessed concurrently
from IOCTLs and IRQ handler. The patch increases protection range of irqlock
to those fields also.

Signed-off-by: Andrzej Hajda <a.hajda@samsung.com>
Signed-off-by: Kamil Debski <k.debski@samsung.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>