MLP DSP: use int16_t instead of int32_t for coeffs

The decoding in read_filter_params insures that coeffs fit within 16bits.

Benchmarks for 2 samples on Arrandale on the whole filter_channel loop:
         A,win32  A,win64    B,win32  B,win64
before:    6110     3172       1870     1068
after:     6100     3160       1862     1026

Aligning the coeffs buffer may also insure that a single line of cache
is accessed, but this had no measurable effect.
MLP: merge FIR and IIR states and padd buffers

In particular, merging and padding their state buffers avoid some back and
forth copying of data, which can also be optimized as 3 128bits moves.

Benchmarks for 2 samples on Arrandale on the whole filter_channel loop:
         A,win32  A,win64    B,win32  B,win64
before:    6100     3160       1862     1026
after:     6020     3125       1843     1010
MLP: align state and coeff buffers

The goal is:
- for state, to allow aligned copies.
- for coeff, to make sure they don't span over cache lines.

Benchmarks for 2 samples on Arrandale on the whole filter_channel loop
         A,win32  A,win64    B,win32  B,win64
before:    6020     3125       1843     1010
after:     5995     3104       1819      995