media: venus: dec: Fix handling of the start cmd

The decoder driver should clear the last_buffer_dequeued flag of the
capture queue upon receiving V4L2_DEC_CMD_START.

The last_buffer_dequeued flag is set upon receiving EOS (which always
happens upon receiving V4L2_DEC_CMD_STOP).

Without this patch, after issuing the V4L2_DEC_CMD_STOP and
V4L2_DEC_CMD_START, the vb2_dqbuf() function will always fail, even if
the buffers are completed by the hardware.

Fixes: beac8290 ("media: venus: make decoder compliant with stateful codec API")

Signed-off-by: Michał Krawczyk <mk@semihalf.com>
Signed-off-by: Stanimir Varbanov <stanimir.k.varbanov@gmail.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>