[media] mtk-vcodec: use V4L2_DEC_CMD_STOP to implement flush

Also remove the code using size-0 OUTPUT buffer to flush.

Signed-off-by: Tiffany Lin <tiffany.lin@mediatek.com>
Signed-off-by: Wu-Cheng Li <wuchengli@chromium.org>
Reviewed-by: Kuang-che Wu <kcwu@chromium.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>