dirac: Only use MMX if MMX is available.

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
dirac/x86: Fix handling blocksizes that are not a multiple of 4

Fixes out of array accesses

Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>