xf86-video-intel-2.10 and newer require libxcb-1.5 and xcbproto-1.6 to enable XvMC. Please add these as conditional (dependent on new xvmc USE flag) dependencies to -2.10.* and -9999. Also, $(use_enable xvmc) needs to be added to CONFIGURE_OPTIONS.
Fixed without a USE flag, let's always enable it. Besides, only a couple apps actually have proper XvMC support, this should break anything. Thanks :)