[media] v4l: vb2: Set data_offset to 0 for single-plane output buffers

Single-planar V4L2 buffers are converted to multi-planar vb2 buffers
with a single plane when queued. The plane data_offset field is not
available in the single-planar API and must be set to 0 for all output
buffers.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Acked-by: Marek Szyprowski <m.szyprowski@samsung.com>
Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Signed-off-by: Sylwester Nawrocki <s.nawrocki@samsung.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>