v3d,freedreno: Introduce a DRM shim for no-op on v3d andfreedreno, or calling out to the v3d simulator.

The goal is to enable testing of parts of drivers without depending on any particular kernel version or hardware being present.

Simply set LD_PRELOAD=$PREFIX/lib/libv3d_drm_shim.so in your environment, and we'll fake a /dev/dri/renderD128 (or whatever the next available node is) using v3dv3. That node can then be used with the surfaceless or gbm EGL platforms.