media: video: Prevent VCAP from dequeue buffers once turned off

VCAP should not report any buffers ready to be dequeued or allow
for any buffers to be dq once streamoff has been called.

CRs-Fixed: 398896
Change-Id: I2eccf62b7ddff0f7cc0343c8e26bd699b335b9dc
Signed-off-by: Terence Hampson <thampson@codeaurora.org>