If you cashed out an RRSP to buy a home in 2017, technically you are only required to start repaying into the RRSP in the year following the year that you cashed out the RRSP (in this case, in the 2019 taxation year).

CRA will not know about you cashing out the RRSP amount until after you file and your 2017 return is assessed.

However, if you did contribute to an RRSP in 2017 and want to have that amount applied to the HBP repayment, you should report the RRSP contribution in your 2017 tax year BUT not claim it as a deduction (in other words, carry it forward to be applied to future years). Once CRA has the HBP repayment information set up in their system, you can then use the undeducted RRSP amount carried forward from a previous year and apply it (designate it as a HBP repayment) as a HBP repayment in the later year.