This commit changes SSSE3 optimized idct8x8 functions to work with
highbitdepth build.

With this commit and the previous one that enabled SSSE3 idct32x32
functions, tests showed virtually no difference on decoding speed for
file fdJc1_IBKJA.248.webm for the build with -enable-vp9-highbitdpeth
option and the build without the option.

Change-Id: Ibe0634149ec70e8b921e6b30171664b8690a9c45