[media] ivtv: fix sparse warning

Fix this warning:

drivers/media/pci/ivtv/ivtv-irq.c:418:9: warning: context imbalance in 'ivtv_dma_stream_dec_prepare' - different lock contexts for basic block

sparse didn't quite understand the locking scheme, so rewrite it to keep
sparse happy.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Cc: Andy Walls <awalls@md.metrocast.net>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>