PISTA1:
He is probably using a digital LCD interface here (yeah, likely the memory-interface). This basically means, that the SOC can drive it independently of the OS, pushing framebuffer data with a configurable clock towards the digital pin.
You can most likely free up pins by reducing the bits you allocate to the DAC-Resistor ladder and mux the least significant bit pins to a different purpose.
Wen using an external dac that gets fed by some sort of bus would need driver support and it probably is nontrivial as soon as accelerated graphics etc. come into play.
uz to chapu a je to pekne vychcany.