Revert "[media] vb2: fix bytesused == 0 handling"

This reverts 'commit 8a75ffb8
("[media] vb2: fix bytesused == 0 handling")'

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: I42735d39f3bb08d031b6130e385f20115fbcb8f7
Signed-off-by: Arun Menon <avmenon@codeaurora.org>