Speed up video memory write access

I have PCIe graphics card connected to HiFive Unmatched. I measured memory set speed (set 32 bit per step) to GPU memory mapped by PCI BAR register and I get about 70 MB/s. 474.6 MB/s is needed for 1920x1080 32 bit 60 Hz display.

Any ideas how to speed up GPU memory access? Use PDMA? x86 has MTRR registers that allow to enable CPU cache for MMIO memory.