V4L/DVB (8088): ivtv: yuv decoder lock fix

write() operations to the yuv device may not always 'start' the decoder.
Although yuv output will be correct, some functions which should be disabled
while the decoder is in use remain enabled. This can result in hardware
registers being corrupted, causing problems with the mpeg decoder. This patch
simply moves the existing decoder 'start' sequence to earlier in the write()
handler.

Signed-off-by: Ian Armstrong <ian@iarmst.demon.co.uk>
Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>