media: vivid: fix wrong sizeimage value for mplane

In several places a division by fmt->vdownsampling[p] was
missing in the sizeimage[p] calculation, causing incorrect
behavior for multiplanar formats were some planes are smaller
than the first plane.

Found by new v4l2-compliance tests.

Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>