piglit dma buf tests hardcoded to use first card and render node

I have a laptop with optimus, which means that the first card and render node are Nvidia and the second uses Intel. If I want to run piglit dma buf tests on the Intel gpu (e.g., with MESA_LOADER_DRIVER_OVERRIDE=i965, or if I have not built nouveau), I have to manually edit piglit_drm_get_driver and change the /dev nodes.

The current solution seems non-optimal, but I'm not sure what a better way to handle this would be.