Using a voltage divider on LED IC driver output
A resistor (~350 Ohms, use 330 Ohm standard value) on each current sink pin, in series with the LED and then to Vcc (12 Volts) will work fine. 

The resistors will drop part of the surplus voltage, thus decreasing voltage across (and dissipation within) the LED controller IC. This will not affect the TLC5947 operation, as each sink line within the IC uses a constant current regulator referenced against Iref: Refer to Functional Block Diagram on page 5 of datasheet. Thus the current sink function is essentially independent of voltage Vo, up to 30 Volts.

Using a single high wattage resistor tied to Vcc to drop voltage, however, may result in instability, as the voltage Vo seen by the sink lines will vary depending on Vf of individual LEDs as they get turned on, and also by number of LEDs on at any time. 

Notes:


Sufficient headroom is required over the LED forward voltage, for the TLC part to regulate current effectively. For calculation convenience, take this minimum headroom to be 1.7 Volts (+ Vf 3.3 Volts = 5 Volts).
The individual resistors must be rated to dissipate sufficient power, i.e. for 20 mA and (12 - 5=) 7 volts drop, 1/8 watt resistors will be insufficient, 1/4 watt will be required.