[media] davinci: vpif_capture: fix start/stop streaming locking

Video capture subdevs may be over I2C and may sleep during xfer, so we
cannot do IRQ-disabled locking when calling the subdev.

The IRQ-disabled locking is meant to protect the DMA queue list
throughout the rest of the driver, so update the locking in
[start|stop]_streaming to protect just this list, and update the irqlock
comment to reflect what it actually protects.

Suggested-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Kevin Hilman <khilman@baylibre.com>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>