Revert "[media] vb2: if bytesused is 0, then fill with output buffer length"

This reverts 'commit 61bd8fb3 ("[media] vb2: if bytesused is 0,
then fill with output buffer length")'

This change is needed to support zero filled len on
Output plane. Without this change, v4l2 framework sets
bytesused to alloc len, in case of zero filled len from client,
causing the firmware to read corrupt data and crash.

Change-Id: I75c53900bdf63b7ee4f3a4768df80415d4b84f8e
Signed-off-by: Arun Menon <avmenon@codeaurora.org>