Fix the lhs/rhs get_offset method

- Previously, we were dividing m_idx/n_idx by m_step/n_step. However, since m_step/n_step can be different from mr/nr, we should divide by mr/nr instead

Signed-off-by: Gian Marco Iodice <gianmarco.iodice@arm.com>

Signed-off-by: Anitha Raj <anitha.raj@arm.com>

Reviewed-by: Anitha Raj <anitha.raj@arm.com>
Reviewed-by: Emil Ohlsson <emil.ohlsson@arm.com>
Approved-by: Emil Ohlsson <emil.ohlsson@arm.com>