V4L/DVB (4323): [budget/budget-av/budget-ci/budget-patch drivers] fixed DMA start/stop code

Fix bug reported by Andrew de Quincey:
After cold boot the saa7146 DMA did not start if the demuxer was opened
before the frontend has locked to the signal.
DMA transfers will be started now if (and only if)
the frontend is locked and data should be sent to the demuxer.

Signed-off-by: Oliver Endriss <o.endriss@gmx.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>