Mesa: Fix UVD Performance (makes 1080p60 / p50 work on radeon uvd systems

Thanks to ckoenig once and again. Some figures: http://forum.xbmc.org/showthread.php?tid=174854&pid=1618470#pid1618470

qvdpautest is not a really optimized benchmark. Real world samples

Famous Basketball Sample and The Singer are now working flawlessly