util/dma_buf: allow custom rendernode
Piglit test based on dma-buf may not finish properly due to wrongly selected rendernode. Patch adds possibility to override selection by MESA_LOADER_DRIVER_OVERRIDE as primary order and by WAFFLE_GBM_DEVICE as secondary. At this moment done for Intel only, otherwise default will be used (renderD128 or card0).
-v2: added WAFFLE_GBM_DEVICE (Marek Olšák)