staging/vc04_services/codec: Clear last buf dequeued flag on START

It appears that the V4L2 M2M framework requires the driver to manually
call vb2_clear_last_buffer_dequeued on the CAPTURE queue during a
V4L2_DEC_CMD_START.
Add such a call.

Signed-off-by: Dave Stevenson <dave.stevenson@raspberrypi.com>