The 5V LCD will not work for 3.3V. Generally, the contrast pin should have a voltage that is 4V below the VCC for the LCD to have some contrast. When the Vcc is 3.3V, to have a 4V difference between the contrast pin and Vcc, you need a -ve voltage. The 5V LCD modules will not have this. There are two solutions to this.
1. Buy an LCD display that is rated for 3.3V. These displays have internal circuits that generate the necessary -ve voltage for contrast.
2. Generate a -ve supply using a -ve charge pump and connect the LCD contrast potentiometer between Vcc and the -ve supply. Check out the attached circuit. The circuit takes a square wave and produces a -ve voltage. You can place a PWM inside PSoC, configure this for 10KHz frequency and connect this output to the charge pump. Then use a potentiometer between VCC and the -ve voltage and connect the center point of this pot to the Contrast pin of the LCD.
Charge Pump.png 8.0 K