media: s5p-mfc: Fix to handle reference queue during finishing

On receiving last buffer driver puts MFC to MFCINST_FINISHING state which
in turn skips transferring of frame from SRC to REF queue. This causes
driver to stop MFC encoding and last frame is lost.

This patch guarantees safe handling of frames during MFCINST_FINISHING and
correct clearing of workbit to avoid early stopping of encoding.

Fixes: af935746 ("[media] MFC: Add MFC 5.1 V4L2 driver")

Cc: stable@vger.kernel.org
Cc: linux-fsd@tesla.com
Signed-off-by: Smitha T Murthy <smitha.t@samsung.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>