vc1dsp: introduce cases for 8x8 and 16x16

This allows further unrolling the DSP implementation where possible.

x86 and ARM DSP modified by simply moving the multiple calls from vc1dec
to the DSP code. Decoding improvements should only occurs because of the
compiler actually able to unroll more.

Decoding time: ~8.80s -> 8.64s (ie around 2%)

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
avcodec/arm/vc1dsp_init_neon: fix code so it compiles and passes fate-vc1

The original patch  seems to be missing a 16x16 function though

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>