NV_PRAMDAC_900
uint32_t reg900 = NVReadRAMDAC(dev, nv_crtc->index, NV_PRAMDAC_900);
NVWriteRAMDAC(dev, nv_crtc->index, NV_PRAMDAC_900, reg900 & ~0x10000);
reg900 = NVReadRAMDAC(dev, head, NV_PRAMDAC_900);
NVWriteRAMDAC(dev, head, NV_PRAMDAC_900, reg900 | 0x10000);
NVWriteRAMDAC(dev, head, NV_PRAMDAC_900, reg900 & ~0x10000);