Basically, no. There simply is no such thing as an open GPU. It's a shame too, because I'm at a point in my life when I'd really like to embrace a completely open hardware platform as my primary desktop, even if it isn't all that powerful by modern standards.
All MCUs are basically micro PCs that have SRAM, flash memory and run internal, binary code, so no, it is not possible. It is, however, possible to have everything open source from bootloader and up, if you don't need advanced graphics or power saving features.