Nouveau NV50 X-Video, EXA Comp. Support

While Nouveau's Maarten Maathuis is busy working on NV50 kernel mode-setting support, Ben Skeggs has been working on a variety of 3D / video features for the NV50 class GPUs. Committed to their development repository tonight is NV50 X-Video support, tile off-screen pixmaps, EXA composite support, and cleaning up some of the code define statements. With this support being all reverse-engineered as NVIDIA is against an open-source strategy the level of support may vary between graphics processors and some of the functionality may be broken or not fully implemented (yet).